Bernie was back but this time Margaret Rebik was in two minds.

In 2016, the retired slaughterhouse worker’s vote helped deliver Marshall county for Bernie Sanders against Hillary Clinton in the Iowa caucuses. Sanders very nearly won the state.

But now, as she watched Sanders relaunch his campaign in Iowa following his heart attack earlier this month, Rebik said she was also eyeing Elizabeth Warren.

“It’s a real toss-up,” she said. “It really is. I met Bernie a long time ago before he even ran, at a private dinner in Iowa City. I remember listening to him speaking and thinking, ‘What he says is what he really wants to do.’ He’s been saying the same thing for years. He don’t change. Anybody that believes in the same thing for so many years is true.

“But I’ve been checking out the other guys too, listening to them talk and all. I don’t really see any difference with Warren. I like her too.”

Rebik is a little embarrassed to admit it – her daughter works for the Sanders campaign in Iowa.

This is the challenge for the Democrat’s most radical candidate with the caucuses just four months away. Sanders needs to win Iowa to position himself as the defining alternative to the national frontrunner, the former vice-president Joe Biden, as the primary unrolls. But now he not only has to allay concerns about his health and age, but must stave off those regarded as interlopers by his most loyal supporters.

Defined against a single establishment candidate in Clinton, Sanders came within a whisker of beating her in Iowa, firing up his campaign.

Margaret Rebik voted for Sanders in 2016, but she is unsure who she’ll vote for now. Photograph: Chris McGreal/The Guardian

A poll for the Des Moines Register last month showed that now distinctions with other candidates are not so stark, and with new concerns about his age and health, some Sanders supporters are bleeding away.

In Iowa, nearly a third of those who caucused for him in 2016 say they will back Warren. About 12% will vote for Pete Buttigieg. The poll put Warren in first place among likely caucus voters, ahead of Biden with Sanders slumping to third.

Still, the same poll showed Sanders supporters are far more enthusiastic about their man than Warren’s backers are about her, suggesting that at least some can be won back.

At a post-heart attack comeback rally in Marshalltown, the first stop on an “end corporate greed” tour, there was bafflement that anyone looking for a progressive candidate would not stick with the man who has set much of the agenda.

“He’s the only candidate who’s been fighting for us consistently his entire career. I feel I can trust him,” said Thomas Thurston, a 33-year-old landscape designer.

Thurston struggled to explain why Sanders has fallen in the polls, other than to question the polls themselves and to say the senator is up against a hostile media.

Sanders reminded his audience that the policies he espoused in 2016 were regarded as “radical and extreme”. Now they are at the heart of the race and his rivals have embraced many in some form, particularly in widening access to public health insurance through the federal Medicare programme. Sanders and Warren want a single public insurance system. Others propose it as an option alongside private schemes.

But Warren has prevaricated over some of its workings, reinforcing doubts that she is as committed as Sanders.

He’s the only candidate who’s been fighting for us consistently his entire career. I feel I can trust him Thomas Thurston

Healthcare reform is important to Thurston and his wife, Kelly, an estate agent, because they are self-employed and say they pay high insurance premiums.

“Warren isn’t as clear about it as he is. I trust Sanders,” said Thurston.

Pat Copley, a 72-year-old former hospital ward clerk who is unflinching in her support for Sanders, said she was drawn to him by his healthcare reforms. But she is uncertain about how the issue plays in small-town Iowa.

On the one hand, private insurance premiums have surged while small hospitals have been closing across the midwest. On the other, she is frustrated that those with good and affordable insurance, particularly labour union members, are fearful of a government-run programme paid for through taxation.

“I know retired union people who are fearful and I don’t know what he can say to change their minds,” she said. “There’s a lot of fears about healthcare. They’re false.”

‘Still shopping around’

Sanders still has several advantages. Endorsements by two members of “the Squad”, representatives Alexandria Ocasio-Cortez and Ilhan Omar, shored up his bona fides as the original radical in the race. He also has the largest election chest filled from small donations by individual supporters and an activist base far more organised and energised than many of his rivals.

Sanders has sought to diminish concerns about his health by casting his age as experience. Still, Copley worries that his heart attack has potential supporters looking elsewhere.

“I hear that talk in Iowa,” she said. “My 78-year-old friend doesn’t want to vote for him because he’s too old. I’m going to continue to vote for him because he speaks to me. It may be pie in the sky but I’m going to follow him,.”

For Alex Abbe, a 32-year-old high school teacher, it is a major issue.

“I’m still shopping around. Honestly, the age and the heart attack concerns me. I don’t plan on caucusing for Bernie,” he said, even though he turned out to see Sanders speak.

Bernie Sanders shakes hands with supporters during a campaign rally at the Iowa state fairgrounds in Des Moines. Photograph: Scott Morgan/Reuters

“I’m attracted to someone who brings energy. I don’t get that with Biden. Elizabeth Warren I do. I’m one of those voters who like the more progressive plans of some of the candidates. But then on the flip side, you’ve got to ask yourself, are those achievable things? I don’t know what route I’m going to go down.”

The downturn in Sanders’ political fortunes has some of his most ardent supporters pondering what to do if he doesn’t win the nomination. Some refused to vote for Clinton last time. Copley did, reluctantly.

“I felt like I had a gun to my head. I don’t know if I can do it again, vote for any old liberal. It would be very hard to vote for Biden. I would not have any regrets voting for Warren. I just hope she wouldn’t turn out to be like Obama. I thought he was going to be a lot more progressive than he turned out being. He did a lot of things I thought were horrid. But Sanders I trust.”

The Thurstons are also conflicted. Both refused to vote for Clinton in 2016. Kelly wrote in Sanders on the ballot paper. Thomas voted for the Green candidate, Jill Stein.

This time Kelly said removing Donald Trump is the priority. Thomas said he “probably” will vote for the Democrat.

“Depends on which one,” he said. “They’re all more progressive now because of Bernie. The options have gotten better. Whether they’re really being true to their word or not we’ll find out.”