Massachusetts Senator Elizabeth Warren raised her hand as one of the only Democratic presidential contenders willing to abolish her own private health insurance in favour of a government-run plan, demanding "structural change" in the economy and the government as Democrats met on the debate stage for the first time in the 2020 presidential season.

Warren's position highlighted a rift within her party's most ambitious contenders over how to approach inequality in America in a prime-time meeting that marked the unofficial starting line for the Democratic Party's quest to wrest the White House from Donald Trump and deny him a second term.

Most of Warren's rivals, including former Texas Representaive Beto O'Rourke, called for universal healthcare, but also favoured preserving the private insurance market.

Vermont Senator Bernie Sanders, who will be in a second debate group tomorrow, has proposed a "Medicare for All" system without private insurance, and Warren said she agreed with him.

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No one on stage attacked Warren by name in the early minutes of a largely civil prime-time clash in which most of the candidates leaned into their party's focus on class warfare.

"I think of it this way. Who is this economy really working for? It's doing great for a thinner and thinner slice at the top," Warren declared. "That is corruption pure and simple ... and we need to make structural change."

New York Mayor Bill de Blasio, who joined Warren in raising his hand, cast the debate as one for "the battle for the heart and soul of our party."

While the crowded field has been courting voters in key states for several months already, the vast majority of the nation has yet to pay close attention to the diverse field.

That began to change today as a collection of 10 candidates, led by Warren, faced each other on national television for two hours.

The overall field is so large that a second group of 10 Democrats, led by early front-runner Joe Biden, will debate 24 hours later.

The groupings were chosen at random by debate host NBC.

Democrats are unified in their deep desire to beat Trump but divided on what kind of candidate is best positioned to do so.

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On one side: candidates like Warren who are demanding dramatic change that includes embracing liberal policy priorities like free universal health care, debt-free college, a forgiving immigration policy and higher taxes on the rich.

On the other: pragmatic-minded Democrats like Biden — and little-known former Maryland Representative John Delaney — who are calling for modest policy solutions that could ultimately attract bipartisan support.

Beyond Warren, O'Rourke and de Blasio, the slate featured senators Cory Booker of New Jersey and Amy Klobuchar of Minnesota, Representatives Tulsi Gabbard of Hawaii and Tim Ryan of Ohio, along with Washington Governor Jay Inslee, former Maryland Representative John Delaney and ex-Obama Housing Secretary Julián Castro.

Trump, the elephant not in the room, was in the air traveling to Japan for a round of trade talks as Democrats faced the nation for the first time in the 2020 campaign.

Earlier in the day, he confirmed that he would watch the debate from Air Force One. His first tweet of the night: "BORING!"

-AP