Democratic presidential contenders battled over healthcare coverage and border policy on Wednesday during a surprisingly heated first debate that laid bare the party's divisions on whether to abolish private insurance and shift to a Medicare-for-All system.

In the first round of back-to-back debates, several of the lesser-known candidates vied for attention in the crowded race to take on President Donald Trump, shouting over one another to grab the spotlight and prove they are capable of standing up to the Republican in the November 2020 election.

The Democratic contenders repeatedly attacked Trump, saying his economic policies benefited the wealthy at the expense of working Americans, and calling his border policies heartless.

"On January 20, 2021, we'll say 'Adios' to Donald Trump," said former housing secretary Julian Castro.

But they also turned their fire on each other, most often targeting Beto O'Rourke. The former congressman tangled with Castro, a fellow Texan, on border policy, and New York Mayor Bill de Blasio on healthcare.

The intensity of the exchanges after six months of a relatively mild 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 Democratic field of more than 20 candidates.

No one mentioned front-runner Joe Biden, who will take the stage with top rival Bernie Sanders and eight other candidates in the second debate on Thursday night.

The battle over healthcare began when the candidates were asked to raise their hands if they support eliminating private health insurance. Only US Senator Elizabeth Warren and De Blasio did so, but they quickly challenged the other eight candidates on stage.

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.