The Democratic presidential race is getting hotter.

At a fierce, freewheeling debate Thursday, the 10 leading Democratic contenders clashed over whether transformative progressive proposals on issues from health care to gun control would energize a winning coalition or drive away crucial voters.

Joe Biden, the front-runner at center stage, flanked by Bernie Sanders and Elizabeth Warren, more directly than before accused his two top rivals of advocating a flawed Medicare for All proposal they hadn't explained how to fully pay for and one that would force millions of Americans to leave private health-care plans they like.

They portrayed him as thinking small at a time the nation's challenges demanded thinking big.

The fundamental question: In the fight to evict President Trump from the White House next year, just where should Democrats stand?

Hours earlier, congressional Democrats were in effect wrestling with that same question, on an issue that didn't come up at the debate: impeaching the president. The House Judiciary Committee approved guidelines for an impeachment inquiry while House Speaker Nancy Pelosi indignantly insisted that the resolution didn't actually mean impeachment was underway — a step she worries would imperil House Democrats in competitive districts.

And almost precisely as the Democrats' debate was beginning in Houston, Trump was starting to speak to House Republicans who were meeting in Baltimore, previewing the arguments he will make in the campaign. He was running well past schedule, and spoke for more than an hour, a split-screen that may have been deliberate.

"The country will go to hell," if a Democrat wins the White House, the president declared, saying they were "going to take your money, they're going to take, and very much hurt, your families."

Third Democratic debate:Who were the winners and losers

Rivals stand side by side

Just 10 Democratic candidates qualified for the debate in Houston, held at Texas Southern University. That's still a big crowd, but it was small enough that all the contenders could appear on stage on a single night. Biden, the leader, and Warren, his fastest-rising rival, were face to face for the first time.

"I know the senator says she's with Bernie," Biden said at one pointing, gesturing toward Warren. "Well, I'm for Barack," touting his relationship with former President Barack Obama. In fact, Obama was lauded by just about everybody, even those who want to replace his signature Affordable Care Act with Medicare for All.

Warren first praised Obama, and then defended installing single-payer system.

"The richest individuals and the biggest corporations are going to pay more, and middle-class families are going to pay less," she said.

While Medicare for All has excited base Democratic voters, some contenders made a more vigorous case against it.

"I don't think it's a bold idea; I think it's a bad idea," Minnesota Sen. Amy Klobuchar said.

Pete Buttigieg, the mayor of South Bend, Indiana, criticized it for taking away the option of private coverage.

"I trust the American people to make the right choice for them," he said. "Why don't you?"

On some issues, the differences among the Democratic presidential candidates are getting sharper.

Former Texas Congressman Beto O'Rourke, who has increasingly focused on gun violence in the weeks since a mass shooting in his hometown of El Paso, argued passionately for a mandatory buyback program for assault-style weapons. Gun-rights advocates call that confiscation.

"Hell yes," O'Rourke declared. "We're going to take your AR-15, your AK-47. We’re not going to allow it to be used against a fellow American anymore.” (Moments after the debate ended, the O'Rourke campaign website was offering $30 T-shirts that read, "Hell yes we're going to take your AR-15.")

The line got cheers from the audience but a rebuke from Klobuchar.

"I personally think we should start with a voluntary buyback program," she said, noting that there was broad agreement on other proposals, including expanded background checks on gun buyers.

Debate surprise:Andrew Yang reveals plan for $1,000 a month for 10 more families. Is it legal?

Centrist candidates stake ground

The more centrist candidates — New Jersey Sen. Cory Booker, Buttigieg and Klobuchar — talked about finding common ground. But Warren, Sanders and Julian Castro argued for throwing the long ball.

"But the truth is that our problems didn't start just with Donald Trump, and we won't solve them by embracing old ideas," Castro, the former Housing secretary, said in what seemed to be a slight aimed at Biden. "We need a bold vision." (Castro later accused the 77-year-old Biden, inaccurately, of forgetting something he had said two minutes earlier, an apparent attack on his age.)

"Of the 160 million people who like their health care now, they can keep it," Biden pushed back, accusing Sanders of not explaining the financing behind his plan.

"If you notice, he hasn't answered the question," the former vice president said, raising his voice and his arms. "This is about candor, honesty, big ideas. Let's have a big idea."

Top moments:Harris calls Trump a 'little dude' and other moments from the debate

Biden holds his own

Biden wasn't always commanding, and he was sometimes put on the defensive about his past positions on immigration and race; he risked getting snickers when he made a head-scratching reference to record players. But he was crisper and more energetic than in the first two debates, and he didn't get rattled as Sanders railed against his proposals. Indeed, he questioned Sanders' assertion that workers who have negotiated so-called Cadillac health-care plans would be better off under Medicare for All.

"They're going to give back that money to the employee?" Biden asked, incredulous. "For a socialist, you've got a lot more confidence in corporate America than I do."

Sanders was, well, Sanders, raspy and vocal.

Warren managed largely to stay above the fray, at times disappearing from the debate's back and forth.

Booker and Klobuchar both found their footing, more actively engaged in the central debate than they have been before.

California Sen. Kamala Harris tried to focus on Trump rather than her rivals, at one point likening the president to the "really small dude" in the "Wizard of Oz."

Castro aimed squarely at Biden with jibes that sometimes came across as harsh, even shrill.

Entrepreneur Andrew Yang announced a $120,000 giveaway to 10 people to highlight his distinctive income-support proposal, though campaign-finance experts immediately said it might violate the law.

And at one time or another during the evening, all the candidates talked about themselves, using the opportunity to introduce themselves to voters.

Buttigieg talked about coming out as a gay man in his socially conservative state, not knowing what the impact would be on his political ambitions.

Klobuchar discussed growing up with an alcoholic father. "He said he was pursued by grace and that's what drove me to become a public servant," she said.

Warren recalled her struggles when she was let go from her job as a special-needs teacher because she was visibly pregnant. "And the reason I'm standing here today," she said, "is because I got back up; I fought back."

Meanwhlie, Booker, who has been a vegan for the past five years, was asked whether he would urge the country to do the same as a way to combat climate change. "You know, first of all, I want to say: No," he replied to laughter. "Actually, I want to translate that into Spanish: No."

Ad drops during debate:Republicans air ad about the dangers of 'socialism' and AOC