MIAMI - Democratic presidential contenders battled over healthcare coverage during their first debate on Wednesday, reflecting the party's divisions on whether to abolish private insurance and shift to a Medicare-for-All system.

In a surprisingly heated first round of back-to-back debates featuring 10 candidates each night, several contenders also clashed over border policy as some of the lesser-known candidates vied for attention in the crowded race to take on Republican President Donald Trump in the November 2020 election.

During the debate in Miami, US Senator Elizabeth Warren and New York Mayor Bill de Blasio were the only two candidates to raise their hands in support of eliminating private insurance.

Warren, a leader of the party's progressive wing who has been surging in opinion polls, said private insurance was taking advantage of Americans. She backs a government-sponsored Medicare-for-All approach and criticized those who say it is not politically feasible.

"What they are really telling you is they just won't fight for it. Healthcare is a basic right, and I will fight for it," she said.

But former US Representative John Delaney, an outspoken critic of Medicare for All who supports a universal healthcare approach that includes private insurance, said Democrats should not throw away a system that some Americans are happy with.

"I think we should be the party that keeps what's working and fixes what's broken," Delaney said.

After years of defending former Democratic President Barack Obama's landmark healthcare law known as Obamacare from Republican attempts to repeal it, Democrats have struggled during the campaign to agree on the best approach to fixing it.

Senator Bernie Sanders, which has gained support in Congress, would create a government-operated plan that eliminates private insurance. It is modeled on the Medicare government healthcare program for seniors.

Former US Representative Beto O'Rourke said private insurance was "fundamental to our ability to get everybody cared for," but de Blasio cut him off.

"Congressman O'Rourke, private insurance is not working for tens of millions of Americans when you talk about the copays, the deductibles the premiums - it's not working. How can you defend a system that's not working?"

O'Rourke also had a heated exchange with his fellow Texan, former San Antonio Mayor Julian Castro, over the separation of families and detention of migrants at the southern border.

Castro said he would decriminalize border crossings by migrants, which he said led to the separation of families. He challenged O'Rourke and others to support him.

O'Rourke said that as a congressman he helped introduce a bill that would ensure that those who are seeking asylum and refuge in the United States are not criminalized.

Castro responded: "I'm not talking about the ones that are seeking asylum, I'm talking about everybody else." He accused O'Rourke of not doing his homework.

The intensity of the exchanges, after six months of a relatively mild and dispute-free campaign, reflected the high stakes in what could be a make-or-break moment for some of the contenders struggling to be noticed in the field of more than 20 candidates.

All 10 participants in the first encounter had an opportunity to step out of the shadow cast by front-runners Joe Biden and Sanders, who are scheduled to take the stage on Thursday night.

Warren, gaining strength in opinion polls, was the headliner on the opening night. She was joined onstage by nine other candidates, including US Senators Cory Booker and Amy Klobuchar and O'Rourke.

Washington state Governor Jay Inslee said he was the only candidate on the stage that had passed a public healthcare option and a law protecting a woman's right to reproductive health and health insurance.

That drew a sharp response from Klobuchar.

"There are three women up here who have fought pretty hard for a woman's right to choose," she said, looking at Warren and congresswoman Tulsi Gabbard.

Trump hinted he would not tweet his reactions live during the first debate, which is taking place as he flies aboard Air Force One to Osaka, Japan, for a G20 summit.

But shortly after it began, he could not help himself.

"BORING!" he tweeted.