DETROIT — The leading liberals in the Democratic presidential primary, Senators Bernie Sanders and Elizabeth Warren, strenuously fought back on Tuesday against accusations of making fanciful promises and imperiling the party’s prospects against President Trump, as a group of moderate underdogs sought to slow their momentum in the second round of debates.
On an evening that could have produced explosions in their own political rivalry, Mr. Sanders and Ms. Warren instead formed something of an ideological tag team to defend their shared agenda, above all on health care. Though each is seen as the other’s chief obstacle in the Democratic race, Mr. Sanders and Ms. Warren did not at any point clash directly.
Instead, they battled an array of comparatively obscure candidates who used the debate as an opportunity — and for some of them, likely a last chance — to express alarm about their party’s embrace of immense liberal policy goals, like the creation of a “Medicare for all”-style health care system, Mr. Sanders’s No. 1 issue, and a broad liberalization of the immigration system.
There was former Representative John Delaney of Maryland, who accused Mr. Sanders and Ms. Warren of making “fairy tale” promises; Gov. Steve Bullock of Montana, who lamented liberal “wish-list economics”; and former Gov. John Hickenlooper of Colorado, imploring the left not to overreach and “FedEx the election to Donald Trump.”
Arguing in somewhat subtler terms was Senator Amy Klobuchar of Minnesota, who declined multiple chances to lash her liberal rivals by name but questioned the viability of their progressive stances on health care and education.
“I have bold ideas,” Ms. Klobuchar said, “but they are grounded in reality.”
With Mr. Sanders and Ms. Warren refusing to yield an inch, the debate laid bare the stark choice before Democratic primary voters: whether to embrace an agenda of transformational economic change in an effort to motivate young and nonwhite voters, or to proceed more cautiously by embracing more incremental appeals that could win over moderates.
Both Ms. Warren and Mr. Sanders insisted that their more daring approach would lead Democrats to victory, and depicted their more moderate critics as bereft of ambition and political courage.
Democrats, Mr. Sanders said, had to wage a campaign “of energy and excitement, and of vision,” or risk losing to Mr. Trump a second time.
“We need to bring millions of young people into the political process in a way that we have never seen, by among other things, making public colleges and universities tuition free and canceling student debt,” Mr. Sanders said.
Ms. Warren, likewise, warned the party not to nominate “a candidate we don’t believe in, just because we’re too scared to do anything else,” and in an exchange with Mr. Delaney expressed exasperation with what she portrayed as his electoral doomsaying.
“I don’t understand why anybody goes to all the trouble of running for the president of the United States just to talk about what we really can’t do and shouldn’t fight for,” Ms. Warren said to applause. “I don’t get it.”
The most protracted exchanges of the night, and by far the most substantive ones, concerned Mr. Sanders’s signature proposal to replace private health insurance with a single-payer system of the kind employed in Canada and a number of European countries. Mr. Delaney and the other moderates attacked the proposal from the first minutes of the debate, calling it a politically toxic idea that would void the health care plans of union members and of employees of private businesses.
“We don’t have to go around and be the party of subtraction, and telling half the country, who has private health insurance, that their health insurance is illegal,” Mr. Delaney said.
Mr. Sanders defended his plan ferociously, with periodic help from Ms. Warren. He cast Mr. Delaney and other Democratic doubters as champions of an indefensible system, and argued that opponents of the single-payer format tended to brand it with traits — like costliness and unpredictability — that the American health care system already possesses.
“The answer,” Mr. Sanders said, “is to get rid of the profiteering of the drug companies and the insurance companies, move to ‘Medicare for all.’”
Responding to pointed criticism from Representative Tim Ryan of Ohio about the impact of the single-payer proposal on union members, Mr. Sanders grew testy for a moment: “I wrote the damn bill,” he said.
After being overshadowed in the first debates last month, Mr. Sanders found himself in the spotlight for much of the evening.
Both he and Ms. Warren depicted skeptics of single-payer health care as being in league with the G.O.P.: Mr. Sanders accused a CNN moderator, Jake Tapper, of using a “Republican talking point” when raising questions about his plan, and noted that “the health care industry will be advertising tonight on this program.” In a similar complaint, Ms. Warren urged Democrats to “stop using Republican talking points” on the issue.
There were other signs of impatience on the debate stage. Moderates offered cautionary warnings about how Republicans might brand Democrats in the general election as outside the political mainstream. But Mayor Pete Buttigieg of South Bend, Ind., drew applause with the prediction that Republicans would brand the eventual Democratic nominee as a wild-eyed extremist no matter what policies that person endorsed.
“Let’s just stand up for the right policy,” said Mr. Buttigieg, who has endorsed a liberal health care policy more modest than “Medicare for all.”
While Mr. Buttigieg is not running as a full-throated populist, he encouraged Democrats to embrace a different kind of transformational agenda, pursuing major institutional changes like the abolition of the Electoral College and the admission of the District of Columbia as a state, in order to “reform our democracy in our time.”
In a week in which Mr. Trump has dominated the news cycle with a spree of attacks on a group of minority lawmakers, the contenders roundly condemned his comments. But it was not just the president’s provocations that thrust him into the center of the debate: He also served as a reminder of the stakes in this election and shaped the argument over what course Democrats should take in 2020.
Several of the long shot candidates asked voters to see their arguments against raw populism in the context of electoral strategy. Mr. Bullock mentioned repeatedly that he carried a state, as governor, that voted for Mr. Trump by a huge margin. And Ms. Klobuchar reminded viewers of her electoral success in the crucial Midwest: “I can win in states like Wisconsin and Michigan and Iowa,” she stressed in her closing statement.
Others asked Democrats to trust them to deal Republicans a crushing Electoral College defeat by other means.
“There’s a new battleground state: Texas,” said former Representative Beto O’Rourke of Texas, reminding Democrats of the go-everywhere, meet-everyone approach that defined his near-miss Senate campaign last year.
He returned to the enticing prospect in his closing statement: “Those 38 Electoral College votes in Texas are now in play,” Mr. O’Rourke said, “and I can win them.”
Even Mr. Sanders, who sometimes expresses disdain for political process, emphasized that he is currently leading Mr. Trump in many public polls, including in the battleground states like Michigan.
The harshest counterattack on the moderates, however, may not have come from Mr. Sanders or Ms. Warren.
“I wonder why you’re Democrats,” said Marianne Williamson, the author and spiritualist. “You think there’s something wrong about using the instruments of government to help people.”
If the debate on Tuesday was a relatively high-minded affair, hinging on deep disagreements over policy and electoral strategy, a far more personally charged clash may be looming on Wednesday. Former Vice President Joseph R. Biden Jr. and Senator Kamala Harris of California will meet for the first time since their crackling confrontation in the initial round of debates in Miami last month. Unlike Mr. Sanders and Ms. Warren, they are unlikely to keep their personal rivalry muted.
But on Tuesday the focus was on ideology, and the rifts were persistent. The same center-versus-left divide evident on health care was also on display as the candidates clashed over trade and immigration, and whether some of the proposals offered by the liberal candidates would represent a boon to Mr. Trump.
“We got a hundred thousand people showing up at the border right now,” said Mr. Bullock. “If we decriminalize entry, if we give health care to everyone, we’ll have multiples of that.”
Turning to Ms. Warren, he accused her of “playing into Donald Trump’s hands” for wanting to make illegal immigration a civil penalty and seeking to provide federal benefits to undocumented migrants.
Ms. Warren fired back that “seeking refuge, seeking asylum” is “not a crime.”
A similar crossfire unfolded on trade, as Mr. Hickenlooper and Mr. Delaney accused Ms. Warren of pursuing a trade agenda closer to Mr. Trump’s than to that of the last Democratic president, Barack Obama. Ms. Warren’s agenda, Mr. Delaney said, “would isolate America’s economy around the world.”
“What the congressman is describing as extreme is having deals that are negotiated by American workers for American workers,” Ms. Warren retorted. “American workers want those jobs. And we can build the trade deals that do it.”
The evening seemed to expose the rifts in the party that might ultimately define the Democratic race once the party’s primary field narrows, and Mr. Biden comes face to face in future debates with Mr. Sanders and Ms. Warren. Most of the men criticizing the two populists on Tuesday are at risk of failing to qualify for future debates, leaving them to act as a kind of preliminary stand-in crew for the centrist side of the party’s ideological battle.
Mr. Ryan, another such marginal candidate, at one point delivered something of a cri de coeur for the party’s dwindling constituency of blue-collar men. Democrats risked eliminating that slim part of their base, Mr. Ryan said, if they did not “talk about the working class issues.”
“We’ve talked about taking private health insurance away from union members in the industrial Midwest, we’ve talked about decriminalizing the border, and we’ve talked about giving free health care to undocumented workers when so many Americans are struggling to pay for their health care,” Mr. Ryan said. “I quite frankly don’t think that that is an agenda that we can move forward on and win.”
Yet Mr. Sanders and Ms. Warren, faced with such unsparing criticism, did not back off. Indeed, perhaps the most notable conflict in Detroit was the one that did not happen, as the two populists locked arms rather than trading blows.
Ms. Warren ignored a question about her boasts that she’s a proud capitalist — a barely-veiled contrast with Mr. Sanders, a democratic socialist — and used the prompt instead to talk of her battles with Wall Street. Mr. Sanders even praised his colleague at one point, echoing her tough talk on corporate America.
“Elizabeth,” he said, “is absolutely right.”