Super PAC Attacks Sanders in an Ad. Sanders Raises $1.3 Million in a Day.

Sanders’ campaign said Wednesday that it had raised more than $1.3 million since it began fundraising the day before off a negative ad produced by the super PAC that targets Sanders by name.

The ad, backed by the political action arm of the group Democratic Majority for Israel, argues that Sanders, of Vermont, would be unable to beat President Donald Trump in the November general election, citing his heart attack nearly five months ago and his left-wing ideology as evidence that he would be too risky a choice for Iowa caucusgoers focused on winning back the White House.

The ad was scheduled to begin airing in Iowa on Wednesday, but the Sanders campaign sent an email to supporters Tuesday warning that Sanders was being targeted by negative ads and alluding to an “outside spending group” without citing Democratic Majority for Israel.

By Tuesday evening, Sanders had posted a video on Twitter addressing the ads. “It is no secret that our campaign is taking on the political establishment and the big-money interests, who are now running negative ads against us in Iowa,” he said in the video. “The billionaire class is getting nervous, and they should.”

The Sanders campaign, like many of its rivals, often points to signs of adversity to juice donations, and Sanders’ supporters responded Tuesday with a flood of contributions. The financial windfall underscored a central challenge facing his opponents: Attacking him can help fuel his campaign.

The attack ad, which will run through the caucuses Monday, comes amid rising concern among some Democratic Party elected officials and strategists, who have grown increasingly anxious about the prospect that Sanders could capture the nomination. Several recent polls have shown him leading in Iowa and New Hampshire, the first two nominating contests.

Many Democrats now believe they need to act fast to prevent Sanders from stringing together a series of victories in the early contests, fearing that a wait-and-see approach could end up being too late and too divisive for the party.

Democratic Majority for Israel is focused on raising support for Israel within the party, and the group and its allies worry that Sanders’ questioning of the Democratic Party’s long-standing support for Israel could damage the United States’ historically strong relationship with the country. But Mark Mellman, the group’s president, said it was equally worried about Sanders’ chances against Trump if he were to become the nominee.

In a statement, Faiz Shakir, Sanders’ campaign manager, condemned the ad and called on the rest of the Democratic presidential field to do the same.

“Every single Democratic candidate running for president needs to condemn these kinds of negative attack ads run by dark money groups,” he said. “It does not reflect the values of our party and it is not in the spirit of our issues-driven primary contest.”

He added, “But what our ​fundraising ​numbers show is our supporters know the establishment will do anything to try to stop Bernie, and they’re ready to rally to his side at a moment’s notice.”

The campaign said its fundraising haul came from more than 70,000 donations.

This article originally appeared in The New York Times .