“The ad raises serious questions about his electability in their own words. Health is one of the things that people raised. But in general people say they like Bernie Sanders, they respect him but they say he can’t win,” Mellman said. The Sanders campaign did not immediately respond.

The ad marks the first time Sanders is facing significant attacks related to his hospitalization last fall. Rival Democratic campaigns, wary of prompting backlash from voters, have been notably silent on Sanders' heart attack. And Sanders' spirited debate performances and rigorous campaign schedule since the heart attack have helped him overcome some voters' skepticism.

Sanders said in October that he would release "comprehensive" medical records by the end of the year but later backed off that pledge, instead releasing several doctors' notes. “At this point, I see no reason he cannot continue campaigning without limitation and, should he be elected, I am confident he has the mental and physical stamina to fully undertake the rigors of the presidency,” his personal cardiologist, Dr. Martin M. LeWinter, wrote.

Defying the expectations of many of his rivals, the health scare seemed to revive his struggling campaign rather than kill it.

Sanders has since surged ahead to first place in New Hampshire and into a close race for first in Iowa in the final weeks before those contests, according to recent polls. The early-state surveys and Sanders' apparent consolidation of the left wing of the Democratic Party suddenly have many establishment Democrats considering the reality that Sanders could be the Democratic nominee in 2020. Now, some of those same operatives have been trying to find a way to halt Sanders' rise.

Realizing attacks would almost certainly come his way in the final days, Sanders has been trying to blunt their effect. "We are taking on the Democratic establishment, and all across the country, let me tell you that the big money interests are getting very nervous," he said this weekend in Ames, Iowa. "They’re looking at recent polls in New Hampshire and in Iowa and they’re saying ‘Oh my god, Sanders can win.’”

Or as film director Michael Moore, a Sanders surrogate, told supporters in Iowa this weekend: "The knives are out.”

It's unclear how the ad will play with voters in Iowa but it will likely further motivate many Sanders supporters. "Yep, zero ways, no chance for backfire here at all, jfc," tweeted Patrick Dillon, who previously worked in Barack Obama's White House and with his 2012 reelection campaign.

The new ad does not mention foreign policy — but the super PAC airing it is linked to Democratic Majority for Israel, which launched in 2019 to "strengthen the pro-Israel tradition of the Democratic Party," it said in a statement last January. The group’s board is co-chaired by former Clinton White House aide Ann Lewis and counts veteran Democratic strategist Paul Begala among its members.

For now, the funders of the super PAC are anonymous: The PAC was formed last year and will not have to disclose its donors for the first time until this Friday, four days before the Iowa caucuses.

Mellman said the group was singling out Sanders because of concerns about his electability as well as his record on Israel.

“We have two concerns. One is that that is it is fundamental to beat Donald Trump and it is fundamental to achieving that goal. Second is that Sanders is in a uniquely bad place concerning the U.S.-Israel relationship,” Mellman said.