Liberal firebrands Bernie Sanders and Elizabeth Warren slapped back against moderate rivals who ridiculed their plan to provide Medicare for all Americans during Tuesday night's debate among Democratic U.S. presidential hopefuls, in which lesser-known pragmatists warned against "wish-list economics."

"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," Warren, a Massachusetts senator, said in a critique of her detractors.

One of the moderates, Montana Gov. Steve Bullock, took a swipe at Sanders, arguing working people "can't wait for a revolution."

"Their problems are here and now," he said.

The tug-of-war over the future of the party will decide exactly what kind of candidate Democrats put up against U.S. President Donald Trump in November 2020. It is a high-stakes debate that pits voters' hearts against their heads as they weigh their desire for an aggressive response to Trump with finding a safe choice who's best positioned to win.

WATCH | 'I wrote the damn bill': Bernie Sanders defends Tim Ryan's claim that Sanders can't promise union members that his health care plan will give them better coverage than their existing plan:

U.S. Democratic presidential hopefuls Bernie Sanders and Tim Ryan debate whether 'Medicare for All' would improve health-care coverage for people who already have a plan through their employers or their unions. 1:33

The fight for the political left was just one subplot on the first night of the second round of Democratic debates.

Twenty candidates are spread evenly over two nights Tuesday and Wednesday. The second night of debates will feature early front-runner and former vice-president Joe Biden and California Sen. Kamala Harris. CNN, which is hosting the debates, chose the groupings at random.

In many respects, the debate is only beginning. The Democratic nomination won't be secured until the party's national convention next July in Wisconsin.

Despite the long road ahead, there is 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 altogether — and effectively pushed out of the race — if they fail to reach new polling and fundraising thresholds implemented by the Democratic National Committee.

WATCH | 'You are playing into Donald Trump's hands': Steve Bullock tells Warren that advocating decriminalization of illegal border crossings won't solve the problem at the U.S.-Mexico border:

U.S. Democratic presidential hopeful Seth Bullock attacks rival Elizabeth Warren on immigration policy, accusing her of 'playing into Donald Trump's hands' by proposing decriminalizing illegal immigration, while Warren says Bullock is pushing for politicians to 'ignore the law.' 2:54

Former Texas congressman Beto O'Rourke, who is participating in the first debate, is likely to qualify, even as he tries to stop his sharp slide in the polls. But those especially at risk among Tuesday's lineup include Bullock and former Colorado governor John Hickenlooper, the only governors in the running, and Midwestern natives such as Minnesota Sen. Amy Klobuchar and Ohio Rep. Tim Ryan. Also on stage: former Maryland congressman John Delaney and author and activist Marianne Williamson.

The better-known contenders such as Warren, Sanders and South Bend, Ind., Mayor Pete Buttigieg face a less dire challenge: to stand strong but avoid any major gaffes in a marathon campaign season against Trump that has only just begun.

In targeting Medicare for All, the more moderate candidates consistently sought to undermine the signature domestic policy proposal of the top two progressives on the stage, Sanders and Warren. They 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.

WATCH | 'The dark psychic force of collectivized hatred': Marianne Williamson calls out systemic racism, calling it the 'dark underbelly of American society':

Author Marianne Williamson says the water crisis in Flint, Mich., reflects a systemic racism that is the 'dark underbelly of America' and that the Democratic Party needs to reckon with. 1:04

'Stop worrying' about Republicans

The New England senators are known for their unapologetic embrace of aggressive plans to overhaul health care, higher education, child care and the economy — ambitious and expensive steps that may be popular among many Democrats but give Trump and his Republican allies ample opportunity to cast all Democrats as extreme.

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."

WATCH | 'Take on his enablers in Congress': Pete Buttigieg calls out Republicans for 'supporting naked racism' in the Trump administration:

South Bend, Ind., Mayor Pete Buttigieg challenges Republican members of Congress to stop putting party over country and stand up to U.S. President Donald Trump on issues of racism in the Democratic presidential primary debate Tuesday. 0:54

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

It was the candidates with the most to lose who were most aggressive with their fellow Democrats early on Tuesday night.

"They're running on telling half the country that their health care is illegal," Delaney said.

WATCH | Warren and Maryland Rep. Delaney go head to head on what policies are realistic to run on in the 2020 election:

Massachusetts Senator Elizabeth Warren criticizes Rep. John Delaney for an agenda that is not ambitious enough for a presidential race in the second round of Democratic primary debates. 0:50

"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," Delaney continued. "It will turn off independent voters and get Trump re-elected."

Yet Sanders and Warren did not back down.

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

WATCH | Minnesota Senator Amy Klobuchar questions whether Sanders, Warren and other progressive Democrats can deliver the health care and education policies they are promising:

Minnesota Sen. Amy Klobuchar accuses other candidates of being more worried about winning an argument than winning an election in Night 1 of the Democratic presidential primary debate. 0:46

As candidates look toward the next round of debates in September, those at the bottom of the polls will be looking to shore up their campaign coffers and their support. To qualify for the next debate, which takes place Sept. 12 and 13 in Houston, they need 130,000 unique donors and at least two per cent support in four polls. So far, of Tuesday's group, Sanders, Warren, O'Rourke and Buttigieg have met those requirements.