The ad, backed by Democratic Majority for Israel political action arm, comes as concerns among moderate Democrats mount that Sanders could win Iowa, setting the avowed democratic socialist on a possible path to the nomination. With Sanders leading in some polls before the Feb. 3 caucuses, the spot is the first sign of a still-disjointed effort to stop his rise by the moderate wing of the party.

It also signals a new line of attack against Sanders. For much of the Democratic nomination race, his rivals have focused on his policy positions, arguing that his proposals for “Medicare for All” and free public college are too expensive and unfeasible.

While Democratic Majority for Israel is focused on raising support for Israel within the party, its spot targets the dominant concern among many Democrats in Iowa and elsewhere: beating President Donald Trump.

Featuring six Iowa voters, the ad argues that Sanders would be unable to defeat Trump in the November general election, citing the senator’s heart attack last year, his left-wing ideology and his liberal policy positions as evidence that he would be too risky a choice for primary caucusgoers focused on winning back the White House.

“I like Bernie, I think he has great ideas, but in Michigan, Pennsylvania, Iowa, they’re just not going to vote for a socialist,” a male voter says. “I just don’t think Bernie can beat Trump.”

Another voter adds: “I do have some concerns about Bernie Sanders’ health considering the fact that he did have a heart attack.”

In recent weeks, some Democratic Party elected officials and strategists have grown increasingly anxious about the prospect that Sanders could capture the nomination. Many believe that they have a limited window to thwart any possible rise or streak of victories in the primaries, and that a wait-and-see approach — such as taking action after the Super Tuesday contests in early March — could end up being too late and too divisive for the party.

While there are informal discussions in Democratic circles about starting an organized effort to stop Sanders, so far no group of donors or organizations has stepped up to helm a broader campaign. On Tuesday, Third Way, a centrist think tank, sent an email to several hundred of its supporters in Iowa, encouraging them to highlight positions taken by Sanders that the group argues “will repel swing voters in battleground states.”

Mark Mellman, the president of Democratic Majority for Israel and a longtime Democratic pollster, said his group began developing an ad several weeks ago, as Sanders rebounded from a slide in the polls last fall.

The concerns about Sanders are two-pronged. The group and its allies worry that his questioning of the Democratic Party’s long-standing support for Israel could damage the United States’ historically strong relationship with the country.

If elected, Sanders would be the first Jewish president. Sanders, who does not often speak about his Jewish upbringing, has called for an American policy toward Israel that addresses both Israeli security and a “pro-Palestinian” perspective. He spent time working on a kibbutz in Israel as a young man and opposes the movement, supported by some liberals, to boycott, divest from and sanction the country.

Mellman said his organization was equally concerned about the ability of the liberal septuagenarian to beat Trump, saying Sanders was in a “uniquely bad position” to win a general election.

“For many months people were saying he really didn’t have much of a chance, but you have to look at the data now and say he does have a realistic chance of winning Iowa and potentially the nomination,” he said. “This is the point at which there could be room for someone else to make their case and have a better candidate emerge.”

On Tuesday evening, Sanders tweeted out a video in which he addressed 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 says. “The billionaire class is getting nervous, and they should.”

Faiz Shakir, Sanders’ campaign manager, sent an email to supporters Tuesday warning them that Sanders was being targeted with negative ads in Iowa.

The email alluded to an “outside spending group” but did not name Democratic Majority for Israel by name. It also said the campaign was aware that a super PAC for former Vice President Joe Biden had placed $1 million worth of ads in Iowa this week as well.

“We have a small lead in Iowa heading into Monday’s caucus,” Shakir wrote. “But outside groups are on the attack and hoping to stop us. Bernie needs us all if we’re going to fight back and win.”

Past attacks have boomeranged to Sanders’ advantage. When Hillary Clinton ripped into Sanders in an interview published last week, her comments led to a rallying of support for his candidacy online.

The group made its ad reservations Monday, booking more than $680,000 to air only in Iowa starting Wednesday morning and running just through the caucuses, a significant level of saturation over a short span just before the nominating contest. The buy is heavily weighted toward Cedar Rapids and Des Moines markets, and the group is also airing the ad on cable, including on CNN, MSNBC and ESPN.

News that the group had made reservations was swirling around Democratic circles Tuesday morning, and the Sanders campaign sent a fundraising text message to supporters warning that “an outside group is starting a $700,000 NEGATIVE AD CAMPAIGN against Bernie.” The text asked for $2.70 to help push back.

(STORY CAN END HERE. OPTIONAL MATERIAL FOLLOWS.)

The ads follow a weekend in which Sanders faced an onslaught of attacks from Democratic rivals. Pete Buttigieg sent a fundraising email warning his supporters that “Bernie Sanders could be the nominee of our party,” followed by another email casting doubt on his ability to defeat Trump. Michael Bloomberg, who is skipping the early states to focus on Super Tuesday and beyond, also took a jab at Sanders’ political beliefs, saying that Bloomberg was the only Jewish candidate in the race who “doesn’t want to turn America into a kibbutz.”

One reason for Sanders’ surge in Iowa is the inability of moderates to coalesce behind one candidate. It is unlikely that the negative attacks on Sanders will unite moderates at this point. But it could spur them to caucus out of fear that not doing so could lead to a victory for Sanders, who benefits from a loyal and unwavering bloc of supporters.

Any anti-Sanders effort, however, is also likely to galvanize his supporters, who often rush to his side at any sign of adversity.

This article originally appeared in The New York Times .