DETROIT — Ambitious proposals for health care, climate change and other policies backed by liberal Democratic presidential contenders came in for sharp critiques from a cadre of moderate candidates jousting for prominence in the second round of debates here Tuesday night.

Sens. Elizabeth Warren (D-Mass.) and Bernie Sanders (I-Vt.), positioned at the center of the stage at Detroit’s Fox Theatre, faced a barrage of challenges from other candidates vying to make a name for themselves while pushing back against the party’s leftward shift.

Warren and Sanders are promoting “bad policies” and “impossible promises,” said former congressman John Delaney (Md.), the first candidate to mention them by name in what would become a string of attacks throughout the night.

“I share their progressive values, but I’m a little more pragmatic,” former Colorado governor John Hickenlooper said, referring to the two liberal senators.

Sen. Amy Klobuchar (Minn.) described her own policies as “grounded in reality.” Montana Gov. Steve Bullock warned against “wish-list economics” and said struggling teachers and farmers “can’t wait for a revolution,” an implicit reference to Sanders.

Warren and Sanders were emphatic in defending their proposals — and each other — joining forces to brand their moderate detractors as too timid for the moment.

“We’re not going to solve the urgent problems that we face with small ideas and spinelessness,” she said. “We’re going to solve them by being the Democratic Party of big, structural change.”

2020 Democratic presidential candidates touted policy goals and hyped their performances in the spin room after the second primary debate in Detroit on July 30. (The Washington Post)

Responding to charges that his proposals were unworkable, Sanders said, “I get a little bit tired of Democrats afraid of big ideas.”

Tuesday’s debate kicked off the second round of 12 scheduled Democratic debates, and its sometimes contentious tone underscored the pivotal moment for the historic field of candidates.

Ten more candidates, including former vice president Joe Biden, are scheduled to appear on the stage Wednesday, on the second night of this round of debates hosted by CNN. Yet Biden, whose lead in the polls has been one of the most enduring aspects of the Democratic primary contest, was not mentioned by name Tuesday night. Instead, other moderates were used as stand-ins for some of the ideas he espouses.

Of the 10 candidates on the stage, half are at risk of not meeting the polling and donor thresholds to qualify for the next round of debates in September. The prospect of being left out of future debates heightened the pressure on them to stand out and make what was potentially a last plea for their relevancy in the crowded field.

While the candidates engaged in tense but mostly academic verbal tussles over issues such as immigration, student loans, climate change and racial disparities, the subtextual issue of electability quickly rose to the surface.

The battle lines were drawn in the first half-hour of the debate, as an impassioned and prolonged exchange broke out over whether to abolish private health insurance in favor of a single-payer system favored by Sanders and Warren. When Sanders was asked how he would respond to Delaney, who described Medicare-for-all as bad policy, the senator offered a blunt retort: “You’re wrong!”

“Why do we got to be the party of taking something away from people?” Delaney asked him in return. “We don’t have to do that. We can give everyone health care and allow people to have choice.”

Warren quickly jumped in, saying candidates who favor Medicare-for-all are not trying to take anything away from the American people.

“That’s what the Republicans are trying to do,” she said. “And we should stop using Republican talking points in order to talk with each other about how to best provide that health care.”

“That is a disaster at the ballot box,” Hickenlooper said of Sanders’s health-care proposals and his support of the Green New Deal. “You might as well FedEx the election to Donald Trump.”

After Sanders noted that he was leading in many polls, Rep. Tim Ryan (Ohio) chided him by referring to the 2016 Democratic nominee. “Hillary Clinton was winning in the polls, too,” he said.

“We do have to win back some of those places we lost and get those Trump voters back if we’re ever going to win,” Bullock said.

“What I don’t like about this argument right now,” Klobuchar said, “ . . . is that we are more worried about winning an argument than winning an election.”

Some of the biggest applause during the night came in response to impassioned statements by self-help author Marianne Williamson, the only person on the stage who has never held elected office.

“The racism, the bigotry and the entire conversation that we’re having here tonight — if you think any of this wonkiness is going to deal with this dark psychic force of the collectivized hatred that this president is bringing up in this country, then I’m afraid that the Democrats are going to see some very dark days,” Williamson said. “We need to say it like it is . . . And if the Democrats don’t start saying it, then why would those people feel that they’re there for us, and if those people don’t feel it, they won’t vote for us. And Donald Trump will win.”

Warren and Sanders, the two liberal stalwarts in the race, appeared on the debate stage together for the first time Tuesday. They largely voiced support for each other’s policies and declined to draw distinctions with one another.

The two have long been ideologically aligned. When Warren was a Harvard professor arguing for consumer protections, she would appear as a guest on the Vermont senator’s radio show, and they met with each other privately before either one announced they were running.

They have voted with each other more than 94 percent of the time since Warren became a senator in 2013, according to records compiled by ProPublica.

Warren has also embraced Sanders’s Medicare-for-all plan, insulating her from a potential attack from him on one of his signature issues.

But the two candidates appear to be in a head-to-head contest for the party’s liberal voters, some of whom have migrated to Warren in recent months as she has unveiled comprehensive plans on a host of issues. Earlier Tuesday, Warren announced a slate of endorsements, including from Rep. Raúl M. Grijalva (D-Ariz.), who was the first member of Congress to support Sanders against Hillary Clinton in 2016.

The debate marked a crucial moment for former Texas congressman Beto O’Rourke, whose campaign has sputtered to the low single digits in polls after launching with a burst of enthusiasm. He has struggled to capture the viral energy of his unsuccessful U.S. Senate race last year in Texas and has seen much of his support drift toward South Bend, Ind., Mayor Pete Buttigieg, who has made a similar argument for a generational shift.

After a widely panned performance in the June debate, O’Rourke has attempted to retool his approach by focusing less on talking points and speaking with more raw passion.

Klobuchar has done little to hide her disdain for candidates such as O’Rourke and Buttigieg, whom she views as having little experience to justify their campaigns.

Tuesday marked a debut for Bullock, who did not qualify for the first debate. He came out swinging, warning Democrats against “wish-list economics” and calling on the party to prioritize U.S. citizens over undocumented immigrants.

Speaking about struggling Americans, Bullock said: “They can’t wait for a revolution. Their problems are in the here and now.”

Several of the more moderate candidates have had trouble distinguishing themselves in a crowded field that includes a former vice president and liberal stars. Some of the loudest voices in the primary race have been from candidates who want to abolish private health insurance, decriminalize unauthorized border crossings and raise taxes, a formula that has raised alarms among moderates.

The debate in the party — on full display Tuesday night — continues the turmoil over what went wrong for Democrats in 2016, and whether the prescription now is to appeal to liberals who didn’t turn out or attempt to win back moderates and independents who voted for President Trump.

Klobuchar, Hickenlooper and Delaney have been among the most vocal in trying to push back against the party’s leftward tilt. But the moderates opened themselves to charges of timidity as they amplified their warnings against bold changes backed by Sanders and Warren.

“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 said, in a biting reference to Delaney.

“I’ve heard some people here tonight — I almost wonder why you’re Democrats,” Williamson said. “You seem to think there’s something wrong about using the instruments of government to help people.”

After a sharp debate over health care, the moderates and liberals engaged in a heated discussion on immigration — including whether to decriminalize unauthorized border crossings and provide taxpayer-funded health care for undocumented immigrants.

Buttigieg, Warren and Sanders said they supported decriminalizing most unauthorized border crossings, arguing that would remove the “tool” Trump was using to separate migrant families.

“We’ve got a crisis on our hands,” Buttigieg said, adding that it wasn’t limited to immigration. “It’s a crisis of cruelty and incompetence . . . It is a stain on the United States of America.”

O’Rourke — who sparred with former San Antonio mayor Julián Castro on the same issue during the first Democratic debate in June — stood by his opinion that unauthorized border crossings should remain a criminal offense. He said that as president, he would work to waive citizenship fees and free “dreamers” from the fear of deportation. After that, however, “I expect that people who come here follow our laws,” O’Rourke said.

The Trump campaign has seized on proposals by Democrats to decriminalize border crossings and offer health coverage to those in the United States without legal documents. The president and his allies have accused Democrats of supporting “open borders” and prioritizing undocumented immigrants over U.S. citizens.

Some Democrats believe that Trump’s argument has resonance in much of America.

When Warren said Democrats needed to be bold rather than allowing Trump to set the terms of the immigration discussion, Bullock offered a quick retort.

“You are playing into Donald Trump’s hands,” he said. “ . . . A sane immigration system needs a sane leader, and we can do that without decriminalizing and providing health care for everyone.”

Foreign policy has often felt like an afterthought in the Democratic primary race, and when it came up toward the latter part of the debate, the candidates illustrated several stark differences over deployment of U.S. troops and the threat of using nuclear weapons.

Buttigieg, who served in Afghanistan, said he would withdraw troops from the country within his first year in office. O’Rourke pledged to withdraw service members from several countries — including Afghanistan, Iraq, Somalia and Yemen — within his first term.

Hickenlooper disagreed, saying such a withdrawal from Afghanistan would trigger “a humanitarian disaster that will startle and frighten every man, woman and child in this country.”

Warren said the United States should pledge not to use a nuclear weapon preemptively, a position that the Obama administration debated but never committed to.

“The United States is not going to use nuclear weapons preemptively, and we need to say so to the entire world,” she said. “ . . . We don’t expand trust around the world by saying, ‘You know, we might be the first to use a nuclear weapon.’ That puts the entire world at risk.”

Bullock pushed back, saying doing so would reduce U.S. negotiating power.

Trump, who responded to the first Democratic debate by tweeting “BORING!” and later handicapped the candidates’ performances, loomed large over the debate stage in Detroit.

The president’s latest string of attacks against minority lawmakers — including a racist go-back-to-your-country taunt this month targeting four congresswomen — came in for heavy criticism from the Democratic candidates.

“We live in a country now where the president is advancing environmental racism, economic racism, criminal justice racism, health-care racism,” Warren said.

Trump’s latest attack, referring to a prominent black congressman’s Baltimore district as a “disgusting, rat and rodent infested mess” and saying “no human being would want to live there,” has set off fresh discussion among Democrats about how to respond to the president’s racially divisive politicking.

Trump’s verbal broadsides targeting Rep. Elijah E. Cummings (D-Md.) have been roundly condemned by Democrats, a number of whom have grown more comfortable calling the president a racist.

In his closing statement, Sanders described Trump as “a racist and a sexist and a homophobe.”

There were few light moments during the debate, which was heavier on policy discussion than personal stories. Williamson took the other Democrats onstage to task for their high-mindedness in discussing issues, saying, “I want a politics that goes much deeper . . . that speaks to the heart.”

She also criticized candidates who had taken money from corporate donors and then promised to side with voters against special interests.

“To think that they now have the moral authority to say, ‘We’re going to take them on,’ I don’t think the Democratic Party should be surprised that so many Americans believe yada, yada, yada,” she said.