Democratic frontrunner Joe Biden and close rivals Elizabeth Warren and Bernie Sanders have clashed over healthcare at the party's third debate.
The debate in Houston, Texas was the first time the three presidential hopefuls faced off on stage.
Only the 10 highest-polling Democrats were eligible to take part.
Former Texas lawmaker Beto O'Rourke drew some of the biggest applause of the night with an impassioned plea for gun control.
Vice-president under Barack Obama, Mr Biden is leading current polls by a considerable margin, with Massachusetts Senator Ms Warren and Vermont Senator Mr Sanders in second and third. The rest of the field are polling in single figures.
The legacy of President Obama came up repeatedly, while there were also questions on immigration, climate change and gun control. But it was healthcare which sparked the fieriest clashes.
The issue of how to reform the US healthcare system has divided moderate and more progressive Democrats.
Mr Sanders and Ms Warren both back Medicare for All, which would expand an existing federally run programme for the elderly to cover all Americans in a single-payer insurance system.
The 10 Democrats taking part in Thursday's debate
Mr Biden criticised Mr Sanders' Medicare for All plan as being too expensive to work and said the country should focus on improving President Barack Obama's Affordable Care Act instead.
"I know the senator [Warren] says she's for Bernie. Well, I'm for Barack. I think Obamacare worked," Mr Biden said. "My plan costs a lot of money…but it doesn't cost $30tn (£24tn)."
Ms Warren defended the policy, saying families have to deal with exorbitant healthcare bills now, and only wealthier individuals and corporations would see increased costs from the plan.
Some of the lower polling candidates also attacked Mr Sanders and Ms Warren, with Minnesota Senator Amy Klobuchar saying that Medicare for All: "I don't think that's a bold idea. It's a bad idea."
Former Texas congressman Beto O'Rourke, whose hometown El Paso was the site of a mass shooting that left 20 dead, was asked whether he supports confiscating assault-style rifles.
"Hell yes we're going to take your AR-15, your AK-47," Mr O'Rourke said to applause. "We're not going to allow it to be used against your fellow Americans anymore."
California Senator Kamala Harris drew President Trump into the debate, saying: "Obviously he didn't pull the trigger, but he's certainly been tweeting out the ammunition."
In their opening remarks most of the candidates attacked President Trump, blasting him for being a divisive leader and accusing him of racism.
Though some sparks flew there was also a lot of common ground between the candidates.
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Here are some of the other memorable moments:
Unlike the two previous debates, Thursday's event took place over one night as only 10 candidates fulfilled the stricter polling and donor requirements. Ten others remain in the race, and four have dropped out.
President Trump said he had no plans to watch the debate but would catch a re-run. He said that Mr Biden was most likely to win if he avoided any "major mistakes".