Senator Bernie Sanders of Vermont raised $25 million in January, his campaign said on Thursday, a staggering sum that gives him an enviable financial advantage at a crucial moment in the Democratic primary race.

He plans to use the windfall to immediately buy $5.5 million in television and digital ads across 10 states, at a time when some of his rivals are shifting or cutting their existing ad reservations.

The $25 million haul is more money than any other candidate raised in any full quarter during 2019, including several presidential hopefuls who hold the big-dollar fund-raisers that Mr. Sanders forgoes. The announcement is the latest sign of an epochal change in money in politics, with candidates now able to finance a top-tier national campaign fueled by masses of donors giving a steady stream of small amounts.

For Mr. Sanders’s top rivals, the news is likely to be unsettling: Their campaigns are all showing signs of financial strain. Former Mayor Pete Buttigieg of South Bend, Ind., has briefly left the campaign trail to raise money, and former Vice President Joseph R. Biden Jr. is taking time next week for a new batch of fund-raisers.

Mr. Sanders can now plan past the New Hampshire primary on Tuesday with greater financial confidence, an important development given the possibility of an extended Democratic primary with multiple candidates including Michael R. Bloomberg, a self-financing billionaire whose full influence will not be felt until March because he is skipping the first four contests.

The Sanders campaign also announced that it had received 1.3 million donations in January, and that more than 1.5 million different individuals had donated over the course of the campaign.

“Working-class Americans giving $18 at a time are putting our campaign in a strong position to compete in states all over the map,” said Faiz Shakir, Mr. Sanders’s campaign manager, in a statement.

The Sanders ad buy includes California and Texas, where the campaign has already been advertising, as well as eight additional states it had not targeted: Arkansas, Colorado, Massachusetts, Minnesota, North Carolina, South Carolina, Tennessee and Utah.

Mr. Sanders’s show of financial might came as the Democratic candidates are still awaiting final results from Monday’s Iowa caucuses, a delay that has had a dampening effect on any one contender developing momentum heading into New Hampshire. Mr. Buttigieg and Mr. Sanders are now all but tied in the Iowa delegate race, with 99 percent of precincts reporting, but the lack of an official winner makes it easier for Mr. Sanders to seize the spotlight with his good fund-raising news.

Both Mr. Sanders and Mr. Buttigieg are competing aggressively, along with other candidates, for a victory in New Hampshire, where Mr. Sanders hopes to establish himself as the liberal front-runner ahead of Ms. Warren (who finished third in Iowa) with a repeat of his big win in the state four years ago against Hillary Clinton. His January fund-raising total outpaced the $21.1 million he raised in January 2016 against Mrs. Clinton.

Despite only having a few days left to campaign before the New Hampshire primary, Mr. Buttigieg spent Wednesday night holding an evening fund-raiser in New York, with another one scheduled for Thursday morning.

A top adviser to Mr. Buttigieg, Michael Halle, wrote a thinly veiled post on Twitter suggesting where a Buttigieg-backing super PAC might best spend its funds, and even alluded to a message that the campaign would appreciate seeing put on television. “Pete’s military experience and closing message from Iowa work everywhere especially in Nevada where it’s critical they see this on the air through the caucus,” Mr. Halle wrote.

Ms. Warren’s campaign manager, Roger Lau, responded by pointing out what he called a “fun fact about how some campaigns exploit our broken campaign finance laws,” suggesting that if Mr. Halle had written that message in private to a super PAC, it could constitute illegal coordination with an outside group.

For her part, Ms. Warren has cut nearly $500,000 in future ad reservations in Nevada and South Carolina over the last two days, according to the media tracking firm Advertising Analytics. Those are the two states that follow New Hampshire in voting in February.

“I just always want to be careful about how we spend our money,” Ms. Warren told reporters in New Hampshire on Wednesday.

Ms. Warren’s campaign told supporters in an email on Tuesday that the Iowa Democratic Party’s inability to announce any results on Monday night had muted whatever financial boost she could have received from the caucuses. “We didn’t get a big night of exciting news coverage about them (or the late-night boost in fund-raising that usually comes with it),” her campaign wrote.

As for Mr. Biden, he began 2020 with the least cash on hand of any of the four top candidates — less than $9 million — and his apparent fourth-place finish in Iowa already had some in the Democratic establishment skittish about his prospects.

His campaign had said ahead of the caucuses that January was his “strongest month of fund-raising since launch.” An analysis of Federal Election Commission records indicates that his January total was somewhere between about $8 million (his previous high in October 2019) and $9.7 million (his April total).

On Wednesday, Mr. Biden was also moving around his ad reservations, with about $150,000 cut from South Carolina and reinvested in Nevada, according to Advertising Analytics. Nevada will hold its caucuses on Feb. 22, and South Carolina has its primary on Feb. 29.

The need for cash is apparent from Mr. Biden’s schedule: In the days after the New Hampshire primary next week, he has no less than four fund-raisers, including two in New York, one in Denver and one in Reno, Nev.

A pro-Biden super PAC, Unite the Country, announced a $900,000 ad campaign in New Hampshire the day after the Iowa caucuses. That would be a sizable share of its remaining funds, according to previous announcements from the group.

Steve Schale, its executive director, had said the super PAC was on pace to spend about $6 million in Iowa, and the group announced on Jan. 31 that it had raised $7.6 million “to date.”

As Mr. Biden and other candidates rush to refill coffers after spending millions in Iowa, Mr. Bloomberg, the billionaire former mayor of New York City who is self-financing his 2020 run, awaits the whole field in March.

Mr. Bloomberg’s presence has acted as something of a potential safe harbor for more moderate donors concerned about both Mr. Sanders’s strong showing in Iowa and Mr. Biden’s weak one.

Reid J. Epstein contributed reporting from Washington.

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