Late last year a group of first-term House Democrats, anxious over the party’s fractious presidential race, convened a series of discussions intended to spur unity. Led by Representatives Colin Allred of Texas and Haley Stevens of Michigan, they considered issuing a collective endorsement of one moderate candidate.

The group held phone calls with Joseph R. Biden Jr., Amy Klobuchar and Pete Buttigieg. But the lawmakers could not agree: Some were torn between the options, and others worried about alienating voters at home who backed other contenders, like Senators Bernie Sanders and Elizabeth Warren. A few issued solo endorsements of Mr. Biden, but the grander plan disintegrated.

“There was not time to reach consensus over one candidate,” said Ms. Stevens, who eventually endorsed Michael R. Bloomberg, recalling the “fast-moving” blur of the lead-up to Iowa.

That effort was just one in a series of abandoned or ineffective plans to rally the moderate wing of the Democratic Party, and the leaders and institutions of the political establishment, behind a single formidable contender who could stop the ascent of Mr. Sanders, a democratic socialist promising a revolution in government.

A gambit to have the powerful Democratic machine in Nevada back Mr. Biden, for instance, fizzled when the former vice president finished fifth in New Hampshire. Two outside spending groups formed to target Mr. Sanders were unable to inflict damage.

Now, on the eve of Super Tuesday, when Democrats across 15 states and territories will hand out more than a third of the delegates required to claim the nomination, Mr. Sanders is within reach of a clear national lead and Mr. Biden is racing to catch up.

In the last few days, moderate Democrats acting with a new sense of urgency have begun a large-scale effort to coalesce around Mr. Biden, with Mr. Buttigieg leaving the race on Sunday and Ms. Klobuchar abandoning her own campaign Monday. In a sign of the new, if frantic, spirit of unity, the two bitter foes both endorsed the former vice president at campaign events on Monday night in Dallas.

But Mr. Sanders has become a formidable front-runner. The Vermont senator has put forth a sweeping set of policy prescriptions — far more ambitious than anything Mr. Biden and other moderates have proposed — to address defining concerns for many Democrats, like economic inequality and the soaring cost of health care and higher education.

His agenda has galvanized liberal voters yearning for change. Mr. Sanders finished atop the first three voting states before trailing Mr. Biden in South Carolina, and polls show him ahead in the delegate-rich state of California on Tuesday.

He holds significant financial and organizational advantages that he was able to accumulate over a long season of disarray among traditional Democrats. And he has begun to make an increasingly direct case to the rest of the party that his urgent message is the best match for a trying political moment, and that he is the candidate most prepared to do battle with President Trump.

Interviews with more than 100 Democratic elected officials, campaign strategists, union leaders and donors revealed a party establishment that spent many months distressed about the implications of nominating Mr. Sanders but frozen in a state of anxiety over who would be the best alternative.

Many of the most influential officials, organizations and donors in the party remain torn between their concern about Mr. Sanders’s chances in the general election, their fear of antagonizing his supporters, and their belated and often-rueful recognition that he has assembled a political movement with appeal well beyond the youthful, left-wing base he built in 2016.

Top Democrats now believe that there are only two realistic paths forward in the presidential race: a dominant victory on Tuesday by Mr. Sanders that gives him a wide lead in the delegate count, or a battle for delegates over months of primary elections, that might allow Mr. Biden to pull ahead or force the nomination to be decided at the Milwaukee convention in July.

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One suburban voter — who has a personal history with Joe Biden — asks himself which moderate candidate has the best chance of beating President Trump.

The mood of emergency among party leaders has been simmering for months but it only translated into a surge of support for Mr. Biden at the very last minute.

“If we go through March 3 and don’t show that we have a nominee who can appeal to a broad swath of our party, we’re going to be in serious trouble,” said Terry McAuliffe, the former Virginia governor and Democratic National Committee chairman, who endorsed Mr. Biden over the weekend.

Even now, authority figures like House Speaker Nancy Pelosi, Senate Minority Leader Chuck Schumer and former President Barack Obama have stayed on the sidelines, convinced their intervention would only fuel Mr. Sanders’s claims of an insider plot against him. Influential labor groups and advocacy organizations have either stayed quiet or supported multiple candidates.

The candidates themselves have made fitful or flawed efforts to garner mainstream Democratic support: A number of lawmakers open to endorsing Mr. Biden said he did little to reach out to them for most of the campaign, letting weeks or months go by without any contact from him or his aides.

A flashy late entrant, Mr. Bloomberg, the billionaire former New York City mayor, initially impressed elected officials with an energetic charm offensive and a war-machine campaign operation. But many Democrats developed serious reservations about him after a damaging debate-stage clash with Ms. Warren last month.

Underdog moderates persisted in pleading with party leaders for help, promising to continue their campaigns even at the risk of playing a spoiler’s role for Mr. Biden or Mr. Bloomberg. Last week, only three days before he dropped out of the race, Mr. Buttigieg visited the Capitol to seek endorsements and vowed in a private meeting with centrist Democrats that he would not withdraw. It was only after South Carolina, when internal campaign projections were clear that they faced a rout on Super Tuesday, that Mr. Buttigieg and Ms. Klobuchar dropped out.

And while Mr. Sanders has made private overtures toward centrists, his harpoons toward “corporate Democrats” and the scorched-earth tactics of his supporters have continued to unnerve Democratic leaders.

Christine Pelosi, one of the speaker’s daughters, last month contacted Mr. Sanders’s advisers to convey her unhappiness that the actress Susan Sarandon, who is backing the Vermont senator, had called for Speaker Pelosi’s ouster, according to a Democrat familiar with the conversations.

Last Wednesday, the speaker herself seemed to have Mr. Sanders on her mind. That day, she declared in a news conference that Democrats would “wholeheartedly support” whomever their voters nominated.

But in a closed-door session in the Capitol with her leadership team, Ms. Pelosi’s irritation was unmistakable, according to two Democratic officials familiar with the conversation.

With Mr. Sanders largely ignoring the roster of bills her majority has passed and boasting that only a true progressive can defeat Mr. Trump, the speaker said House Democrats proved in 2018 how to win elections; she cited their legislative accomplishments and dismissed the “geniuses” on television who claim the party is shifting sharply to the left.

Ms. Pelosi also invoked her own liberal credentials: In her basement, she noted, she still had old campaign signs supporting progressive causes.

It was not only the national Democratic establishment that declined to unite around Mr. Biden or anyone else. Efforts in the states crumbled or proved ineffective: After the muddled results in Iowa and New Hampshire, Mr. Biden’s campaign hoped to make a strong stand in Nevada, the first diverse state to weigh in on the nomination.

Mr. Biden’s advisers privately projected optimism that they could win endorsements from the most important forces in Nevada politics: Gov. Steve Sisolak, a popular moderate; the casino workers union; and Harry Reid, the former Democratic leader in the Senate. But spooked by Mr. Biden’s debilitating loss in New Hampshire, none of them backed him before the caucuses. Mr. Sanders won the state by an overwhelming margin.

As in other states, Democratic leaders said there had been limited outreach by Mr. Biden. In an interview before the caucuses, Mr. Reid said he had not heard from him or his campaign chairman, Steve Ricchetti, in “several weeks.”

Another candidate was more proactive, pursuing Mr. Reid and many other Democratic leaders with solicitousness: Mr. Bloomberg.

Mr. Bloomberg visited Capitol Hill and dispatched aides around the country to seek endorsements and outline his plans for the campaign against Mr. Trump. His advisers have paraded Democrats through their Times Square headquarters. And even though he was not competing in Nevada, Mr. Bloomberg visited Mr. Reid and his wife, Landra, at their home outside Las Vegas.

By early February, Mr. Bloomberg had effectively frozen the support of other moderates, overwhelming voters with advertising and tantalizing Democratic power brokers with his vision for the general election. In an illustration of Mr. Bloomberg’s allure even to maverick Democrats, his advisers reached out to Andrew Yang after he withdrew from the presidential race to offer counsel about a possible campaign for mayor of New York City next year.

Yet Mr. Bloomberg has not always lived up to the image of a can-do executive he presents in his $500 million advertising campaign.

At a private meeting with Democratic governors in Washington, right after the Iowa caucuses, Mr. Bloomberg delivered a casual performance that alternately charmed and puzzled several attendees. He spoke vaguely about how he would unite the party and win over progressives, suggesting he could heal wounds by being kind to his critics in public and sending flowers to their wives, according to multiple people present.

In an awkward exchange, Mr. Bloomberg said he was uncertain why Ms. Warren, a Massachusetts senator, was faring poorly in the race; when Gov. Kate Brown of Oregon suggested gender could be a factor, Mr. Bloomberg said he disagreed. He emerged with no new supporters.

Mr. Bloomberg still secured a stream of endorsements from prominent Democrats. But his campaign lost a number of defectors because of his disastrous first debate, which dented his polling numbers and emboldened his critics.

“You can’t sell Bloomberg to the country,” said Representative Cedric Richmond of Louisiana, a Biden co-chair. “How’s he going to build a base with the stuff he’s said?”

Even as the race moved on from their early-state successes, Ms. Klobuchar and Mr. Buttigieg clung to hope in confounding ways.

Mr. Buttigieg insisted to donors that if he raised enough money, he could pull off an upset at the convention. Ms. Klobuchar was seen as a potential contender after her third-place finish in New Hampshire but, always attuned to her press coverage, she spent time in recent days sending personal messages to media figures complaining about their reporting and boasting of her energetic campaign schedule.

Despite their wariness of Mr. Sanders, party leaders have hesitated to mount an all-out effort against him. A pair of outside-spending efforts targeting him have struggled to get off the ground: Ad campaigns in Iowa and Nevada by a pro-Israel group failed to halt Mr. Sanders’s advance, and a second group, known as the Big Tent Project, has managed to cobble together only a few million dollars.

At a February luncheon for vulnerable lawmakers, Representative Cheri Bustos of Illinois, the chairwoman of the Democratic Congressional Campaign Committee, pressed Democrats not to rule out candidates whom they could feel pressure to support later. Her entreaty came after one vulnerable Democrat, Representative Anthony Brindisi of New York, told his hometown paper he would not support Mr. Sanders or Ms. Warren.

But Ms. Bustos is said to be worried about Mr. Sanders, and her committee plans to conduct polling in swing districts to see how nominating him might affect House races.

At the same time, Mr. Bloomberg has continued stoking that mood of alarm as he bids for, if not the heart, then at least the more calculating head of the Democratic establishment.

In a private briefing in Charleston, S.C., last week, one of Mr. Bloomberg’s senior advisers, Dan Kanninen, made an extensive presentation detailing the risks of nominating Mr. Sanders. “Florida is off the map,” he said, citing Mr. Sanders’s past praise of some of Fidel Castro’s programs, according to one person who shared a detailed account of the meeting.

In the same session, Mayor Muriel Bowser of Washington, D.C., warned that Mr. Sanders had a “cultlike following,” and claimed Mr. Biden was no longer a viable option: “This is a choice between Bernie, Mike and Trump,” she said.

Days later, Mr. Biden won South Carolina by nearly 30 points.

On the eve of Super Tuesday, Mr. Sanders still appears to hold an upper hand, with a lead in national polls backed by his muscular fund-raising machine and grass-roots following. On Sunday, he announced that he had raised $46.5 million in February alone.

Given Mr. Sanders’s history of acrimony with the Democratic establishment, some in the party are skeptical that he can be induced to work cooperatively with party leadership. Some of his advisers are eager to signal that he can, though they calculated they would have a better chance after what they are hoping will be a breakthrough on Super Tuesday.

Mr. Sanders has made efforts toward comity in the past: He met several times last year with more moderate Democrats in Washington, pitching himself as a nominee they could at least tolerate. Meeting with the New Democrat Coalition last year, Mr. Sanders conceded that he had no hope of being their first choice, but said that he hoped they could accept him eventually, according to a lawmaker who heard his pitch.

Last summer, Mr. Sanders dined with another group of House members to make the case that his economic message and outsider’s approach would have wide traction in the general election. “Bernie’s argument is, ‘Don’t pigeonhole me as just a progressive candidate,’” said Representative Dan Kildee of Michigan, who attended the dinner.

But Mr. Sanders has done little over the last month to reassure skeptics within his party, striking a posture of defiance that may have helped accelerate the late movement toward Mr. Biden. Most senior Democrats are hoping that Mr. Sanders’s forward march can be slowed, and some have urged Mr. Obama to intervene.

Mr. Obama has shown no inclination to do so, reasoning that he must hang back to preserve his ability to help unify the party at the end of a messy nominating process.

Mr. Biden’s aides see that argument as so much malarkey, and they have conveyed as much to Mr. Obama’s inner circle, arguing that the former president’s leverage would evaporate once Mr. Sanders accrued millions of votes. They have pleaded with Mr. Obama’s camp to distance him from Mr. Bloomberg, who has run saturation-level advertising showing images of himself with the former president. When a poll came out last week showing that many Democratic voters believe Mr. Obama is supporting Mr. Bloomberg, the Biden campaign shared it with Mr. Obama’s aides.

Party leaders are already preparing for the possibility of a contested convention, and many superdelegates have indicated they are open to selecting a nominee besides Mr. Sanders if he collects the most delegates but falls short of a majority.

Tom Perez, the D.N.C. chairman, has been clear in conversations with allies that he will abide by the current rules and would not resist an effort by superdelegates to determine the nomination on a second ballot. Referring to the number of delegates needed for a majority, Mr. Perez told one Democrat, “1991 delegates are required,” and noted that Mr. Sanders is familiar with the guidelines.

With the identity of the Democratic standard-bearer unknown, Mr. Schumer has begun to ponder ways of uniting the party behind whoever that person might be: The Senate leader is particularly focused on the idea of nominating an African-American woman for vice president, mulling names like Senator Kamala Harris of California, Stacey Abrams of Georgia and Representative Val Demings of Florida, according to people who have spoken with Mr. Schumer.

As long as a contested convention remains possible, that could make it even harder for any single candidate to put together a decisive primary coalition.

Mr. Reid of Nevada said he was hopeful the race would not last until Milwaukee. But if no candidate achieved a delegate majority by the end of primary voting in June, Mr. Reid said last month that he was prepared to step in to arrange a deal before the convention in mid-July. In the run-up to his own state’s caucuses, Mr. Reid reached for a conciliatory role and cautioned Democrats not to put pressure on Mr. Sanders “to tone down anything.”

Mr. Trump, he said then, could be defeated by any of the top Democrats.

But nine days after Nevada Democrats voted, and weeks after his intervention could have resurrected Mr. Biden and slowed Mr. Sanders in the state, Mr. Reid had evidently reconsidered: On Monday, he issued a belated endorsement of Mr. Biden, calling him the best candidate “to defeat Trump and lead our country following the trauma of Trump’s presidency.”

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