“I think that’s a strategy to bleed as many people off of Warren as he can, particularly if she’s fading,” said Joe Trippi, the former campaign manager for Howard Dean’s 2004 presidential campaign. “It’s a lot easier for him to pick off progressives who like Warren for Medicare for All, like him for Medicare for All. You’re not going to win the argument with people who are worried about the private insurance companies being taken away.”

Sanders has also been fighting in recent days with Biden over their records on Social Security, an apparent attempt to sap the former vice president’s strength among senior voters. Both candidates have exchanged negative videos over the issue on social media.

At the same time that Sanders is promoting Medicare for All on the airwaves, the Democratic Majority for Israel super PAC is pouring nearly $700,000 into TV ads in Iowa that claim he cannot beat Trump because of his democratic socialist identity. They also raise the issue of his heart attack, a subject his Democratic rivals have shied away from.

Sanders responded in an online video, using the attacks as an opportunity to underscore his populist message: “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. The billionaire class is getting nervous, and they should.”

That video illustrates another goal of Sanders’ pitch as he faces attacks by opponents on the left and right before the Iowa caucuses: As much as it’s about health care, it’s also about sending the message that he is sticking to his guns even under siege, something his team believes will energize his supporters and motivate them to go to the polls.

"You have Sen. Sanders here in the closing stretch being willing to demonstrate that he has the spine for the fight,” Shakir said.

There is some evidence his approach might be working: The Sanders campaign said Wednesday it raised more than $1.3 million from 70,000 donations after sending a fundraising email to supporters referencing the Democratic Majority for Israel’s negative ad, though not by name, as well as spending by a pro-Biden super PAC.

Mark Mellman, president of the Democratic Majority for Israel, downplayed the fundraising.

“Sanders raised over $100 million for this race,” he said. “So this $1 million doesn’t amount to a hill of beans.”

Shakir did not rule out the possibility that the campaign could air a TV ad in Iowa defending Sanders in the wake of the attacks, including around electability, however. And the Sanders campaign is considering a spot contrasting his record with Trump's, according to a source.

Asked whether another ad was coming in Iowa, spokesman Mike Casca said, “Stay tuned.”

On the stump, Sanders has told Iowans he is the best-positioned candidate to defeat Trump and boasted that the president views him as a threat.

The conservative Club for Growth Action is also airing an ad in Iowa that says the 78-year-old Sanders is “too old” and “too liberal.” But it criticizes him in a way that may appeal to Democratic voters, saying he wants to give “government health insurance to everyone” and comparing him with liberal icons Barack Obama and Franklin D. Roosevelt.

Sanders has supported single-payer health care for decades, and Medicare for All was a key part of his campaign platform in his 2016 presidential run. But his team decided this summer that a renewed focus on health care was critical to jump-starting his campaign. That decision came amid concerns that a different strategy was needed to stand out in a multicandidate field.

In addition to airing ads promoting Medicare for All, Sanders has for months put this strategy to use in town halls in Iowa, where he asks residents to share their experiences dealing with the health care industry. Residents now come to the events prepared to talk about their troubles.

While top Sanders surrogate Rep. Alexandria Ocasio-Cortez was taking questions at a Cedar Falls town hall on Saturday, a woman told her that her paychecks were being garnished because of medical bills. She started crying, and they embraced. At a rally in Marshalltown the same day with Ocasio-Cortez and filmmaker Michael Moore, two people who attended cited health care as a top reason they are supporting Sanders.

“I’m really, really into health care,” nursing assistant Angela Hodge said. “My son works really hard and he can’t always afford everything my grandson needs. … So my grandson does without. It’s almost $10,000 to buy a cochlear [implant] for my deaf grandson.”

Her husband, James, added: “We’ve got a funeral going on right now for a young friend who couldn’t afford his insulin and that’s why he passed away.”