Democratic leader Joe Biden and close rivals Elizabeth Warren and Bernie Sanders clashed for health reasons during the party's third debate.
The debate in Houston, Texas was the first time the three presidential candidates had clashed on stage.
Only the 10 most voting Democrats were eligible.
Former Texas lawmaker, Beto O 'Rourke, drew one of the biggest applause of the night with a passionate call to gun control.
Vice President Barack Obama, Biden is leading current polls, the Massachusetts Senator, Mrs. Warren, and the Vermont Senator, Mr. Sanders, in second and third place. The rest of the field queries a single digit.
The legacy of President Obama was evoked repeatedly, while there were also questions about immigration, climate change and gun control. But it was health care that triggered the most violent clashes.
Why is health care controversial?
The question of how to reform the American health system has divided moderate and more progressive democrats.
Sanders and Warren are both in favor of Medicare for All, which would expand an existing federal seniors program to cover all Americans with a single payer insurance system.
Biden criticized Sanders' Medicare for All plan as too expensive and said the country should instead focus on improving President Barack Obama's affordable care law.
"I know the senator [Warren] says she's for Bernie. Well, I'm for Barack. I think Obamacare worked, "said Biden. My plan costs a lot of money … but it does not cost $ 30 billion (£ 24tn). "
- Can Bernie Sanders Revise US Health Care?
Warren defended the policy, saying families now have to contend with exorbitant health bills, and that only wealthier individuals and corporations would see their costs rise.
Some of the lowest candidates also attacked Sanders and Warren, Minnesota Senator Amy Klobuchar, who said Medicare for all: "I do not think it's a bold idea. . "
What about gun control?
The former Texas Congressman, Beto O 'Rourke, whose hometown of El Paso was the scene of a massive shooting causing 20 deaths, was questioned about his support for the confiscation of assault rifles.
"Yes, we will take your AR-15, your AK-47," said Mr. O. Rourke to applause. "We will no longer allow this to be used against your fellow Americans."
Californian Senator Kamala Harris brought President Trump into the debate, saying: "Obviously, he did not pull the trigger, but he certainly tweeted the ammo."
In their opening remarks, most candidates attacked President Trump, accusing him of being a division leader and accusing him of racism.
What happened on the other?
Although some sparks flew, there was also much in common between the candidates.
Here are some of the other memorable moments:
- Businessman Andrew Yang used his opening speech to announce that he would use donations for his campaign to create a "Freedom Dividend," giving $ 1,000 (£ 810) a month to 10 families. Americans, making other candidates laugh
- A Spanish-speaking moderator asked Senator Cory Booker of New Jersey, a vegan, that he wanted Americans to stop eating meat to fight climate change. He said, "First of all, I mean no, in fact, I want to translate that into Spanish: no."
- Mr. Biden gave a rambling answer about fighting the legacy of slavery by entering education, speaking of keeping a "record player" at night [to] make sure the children hear the words "
- President Obama's Housing Secretary, Julián Castro, responded to criticism from Pete Buttigieg's Mayor, South Bend, in Indiana, that the debates were "unstoppable" by saying, "It's an election. "
- Mr. Castro also attacked Mr. Biden, accusing him of "forgetting what you said two minutes ago" in an exchange on health care
- Immigration rights protesters entered the room at one point, interrupting Mr. Biden while he was answering a question about resilience by talking about the need to deal with the loss of his first wife and her children.
- Buttigieg, the first openly gay candidate in the presidential election, said in an opening statement: "It did not interest me to know what it was to be lover."
Unlike the previous two debates, Thursday's event was one night, with only 10 candidates meeting stricter voting and donor criteria. Ten others remain in the race and four have dropped out.
President Trump said he did not intend to follow the debate, but would catch up. He said Mr. Biden was very likely to win if he avoided "major mistakes".
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