DAVENPORT, Iowa — If Senator Elizabeth Warren thought her proposal to pay for Medicare for All would quiet her critics and rivals, that plan didn’t work.

As Warren and other Democratic presidential candidates descended on Iowa this past weekend, the attacks came flying at her. Former vice president Joe Biden’s campaign suggested she is dishonest. Mayor Pete Buttigieg of South Bend, Ind., questioned her math. And even Vermont Senator Bernie Sanders, who has long refrained from attacking her, said his own plan to fund Medicare for All is more progressive.

Warren holds a slim lead here in the first caucus state, according to a recent New York Times/Siena College poll. But as the biggest weekend of Iowa’s fall campaign season drew to a close, it was clear she will have to fight a two-front battle to hold onto that lead, with Sanders showing a new willingness to knock her from the left and Biden and Buttigieg laying into her from their more moderate positions.

Well aware of those threats, Warren responded more directly to her rivals after weeks of trying to hover above the fray, calling out Biden by name and retooling the end of her stump speech to address lingering questions about her electability.

“There are people who will just as easily give up on a big idea and sound oh so smart and oh so sophisticated,’’ Warren said in a high school gymnasium Sunday, offering a sharpened defense of a candidacy that is betting bold policy ideas will turn out new voters.

But going on the offense may not be enough to lift Warren out of the Medicare for All morass where Biden and others want her mired. As reporters grilled her on her plan on Saturday, Warren made a rare misstep in describing her funding plan, handing her rivals fuel for criticism and dragging her further into the weeds of a health care debate she never seemed eager to have in the first place.

On Saturday night, Warren told a reporter in Dubuque that her plan “doesn’t raise taxes on anybody but billionaires,’’ even though it has provisions that would affect the top 1 percent of American families, many of whom are worth much less than that. Warren’s communications team said she was talking only about a provision of the plan that raises her proposed wealth tax for billionaires, but the Biden campaign pounced anyway.

“The American people have to be able to trust whoever our party nominates to take on Donald Trump to be straight with them about health care,’’ said Kate Bedingfield, the deputy campaign manager for Biden, seizing on the gaffe on behalf of a candidate who had mistakenly said he was in Ohio, not Iowa, just hours before.

Biden’s statement also singled out Warren’s capital gains tax, even though his own health care plan would also increase that tax for wealthy families.

Warren has propelled herself to the top tier of the Democratic presidential field with a soak-the-rich fight against corporate greed and Washington corruption, electrifying progressives with ambitious policy proposals.

But Warren’s support for Medicare for All became a major sticking point as candidates including Biden and Buttigieg knocked her vow to deliver an entirely government-funded health care system that did not raise taxes on the middle class without specifying how. Sanders, who wrote the Medicare for All legislation, which does not detail how the program would be funded, never came to her defense. Warren was left to absorb the attacks on her own.

So Warren arrived in Iowa on Friday with a 20-page white paper laying out her $20.5 trillion plan to fund Medicare for All over 10 years without raising taxes on the middle class by “one penny.’’

It would be paid for, the campaign said, with a 6 percent wealth tax on billionaires — which is higher than the 2 percent tax she already proposed on people with more than $50 million — as well as taxes on financial transactions and capital gains taxes for the top 1 percent.

Employers would have to put about $8.8 trillion into the system, which is similar to their contributions to the current system, and other funding would come from defense cuts and an immigration overhaul.

Biden’s campaign immediately ripped into Warren’s plan on Friday, which prompted her to suggest he was “running in the wrong presidential primary.’’ Meanwhile, Buttigieg took the stage in front of some 13,500 Iowa voters at the Liberty and Justice dinner in Des Moines Friday night and made comments that seemed to suggest she is too divisive for the general election.

“I will never allow us to get so wrapped up in the fighting that we start to think fighting is the point,’’ Buttigieg said.

When it was her turn to speak, Warren accused her rivals of thinking too small — “Anyone who comes on this stage and tells you they can make change without a fight is not going to win that fight,’’ she said — but by Saturday, she was back to defending herself on Medicare for All.

At a town hall-style event in rural Vinton, a voter asked how she planned to transition to Medicare for All, and Warren focused on her funding plan instead of answering the question (her white paper says she will roll out a transition plan in the coming weeks), touting her claims that it would eliminate $11 trillion in Americans’ out-of-pocket medical expenses over the next decade.

Warren’s rise has drawn fierce attacks from candidates who want to be seen as a moderate alternative to her, but her health care funding plan has also provided fodder for Sanders to ding her after months of an apparent nonaggression pact between the two liberals.

At his own event in Cedar Rapids Saturday, he seemed to draw a distinction between Warren’s promise of free health care without a middle-class tax increase, and his own idea, which he said would levy a 4 percent income tax.

“I’m not going to say it’s free,’’ Sanders said. “Nothing is free. Health care’s expensive.’’

He made more pointed comments in an interview with ABC News, saying his approach “will be much more progressive in terms of protecting the financial well-being of middle-income families.’’

Warren’s campaign has pointed out that her plan cuts costs for families at the expense of the wealthiest Americans and corporations. But on Sunday, Warren declined to take the bait, saying simply, “Bernie may have a different vision of how to pay for it, but let’s be really clear, Bernie and I are headed in exactly the same direction.’’

But the issue has put her on the defensive and could distract from her core message at a time when Buttigieg and Sanders are gaining support in Iowa polls. Several voters interviewed at Warren’s campaign events said they were trying to decide between her and either — or both — of them.

“I feel safer with him sometimes,’’ said Elise Crow, a lawyer from Cedar Rapids, of Buttigieg, but she said Warren was persuasive too. “You need the numbers and you need the excitement, so I’m more open to it.’’

Jess Bidgood can be reached at Jess.Bidgood@globe.com. Follow her on Twitter @jessbidgood.