The signature domestic proposal by the leading progressive candidates for the Democratic presidential nomination came under attack from moderates in a debate that laid bare the struggle between a call for revolutionary health insurance policies and a desperate desire to defeat President Donald Trump.

Standing side by side at center stage on Tuesday, Bernie Sanders and Elizabeth Warren slapped back against their more cautious rivals who ridiculed “Medicare for All” and warned that “wish-list economics” would jeopardize Democrats’ chances for taking the White House in 2020.

“I don’t understand why anybody goes to all the trouble of running for president of the United States just to talk about what we really can’t do and shouldn’t fight for,” said Warren, a Massachusetts senator, decrying Democratic “spinelessness.”

Sanders, a Vermont senator, agreed: “I get a little bit tired of Democrats afraid of big ideas.”

A full six months before the first votes are cast, the tug-of-war over the future of the party pits pragmatism against ideological purity as voters navigate a crowded Democratic field divided by age, race, sex and ideology. The fight with the political left was the dominant subplot on the first night of the second round of Democratic debates, which was notable as much for its tension as its substance.

Twenty candidates are spread evenly over two nights of debates Tuesday and Wednesday. The second night features early front-runner Joe Biden, the former vice president, as well as Kamala Harris, a California senator.

The marathon presidential primary season won’t formally end for another year, but there was an increasing sense of urgency for many candidates who are fighting for survival. More than a dozen could be blocked from the next round of debates – and effectively pushed out of the race – if they fail to reach new polling and fundraising thresholds implemented by the Democratic National Committee.

Sanders’ plan to provide free universal health care, known as Medicare for All, has become a litmus test for liberal candidates, who have embraced the plan to transform the current system despite the political and practical risks. Medicare for All would abandon the private insurance market in favor of a taxpayer-funded system that would cover all Americans.

In targeting Medicare for All, the more moderate candidates consistently sought to undermine Sanders and Warren. The moderates variously derided Medicare for All as too costly, ineffective and a near-certain way to give Republicans the evidence they needed that Democrats supported socialism.

“They’re running on telling half the country that their health care is illegal,” said former Maryland Rep. John Delaney.

“We have a choice: We can go down the road that Sen. Sanders and Sen. Warren want to take us, which is with bad policies like Medicare for All, free everything and impossible promises,” he continued. “It will turn off independent voters and get Trump reelected.”

Yet Sanders and Warren did not back down. While they are competing for the same set of liberal voters, there seemed to be no daylight between them.

“Health care is a human right, not a privilege. I believe that. I will fight for that,” Sanders said.

South Bend, Indiana Mayor Pete Buttigieg called on his party to stop the infighting.

“It is time to stop worrying about what the Republicans will say,” Buttigieg declared. “It’s true that if we embrace a far-left agenda, they’re going to say we’re a bunch of crazy socialists. If we embrace a conservative agenda, you know what they’re going to do? They’re going to say we’re a bunch of crazy socialists. So let’s just stand up for the right policy, go out there and defend it.”