The first debate of the 2020 Democratic US presidential race kicked off with a two-hour event featuring 10 candidates, including three senators, a mayor and several current or former members of Congress.

The ten Democrats railed against a national economy and Republican administration they said exists only for the rich, embracing class warfare as a defining theme in their fight to deny President Donald Trump a second term in office.

Health care, more than any other issue, led the debate.

Massachusetts Senator Elizabeth Warren, more than anyone else, stood out — on her own at times — in calling for “fundamental change” across the US’s economy and government to address persistent issues of inequality.

“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,” Ms Warren declared shortly before raising her hand as one of the only Democrats on stage willing to abolish her own private health insurance in favour of a government-run plan.

“Health care is a basic human right and I will fight for basic human rights.”

Election 2020 Debate
Democratic presidential candidate Elizabeth Warren gestures towards Cory Booker during the debate (Wilfredo Lee/AP)

The debate marked a major step forward in the young 2020 presidential campaign as Democrats seek to break out from the crowded field that has so far been dominated by former vice president Joe Biden, who will appear in a second debate featuring another 10 candidates on Thursday night.

Mr Biden was not mentioned during Wednesday’s face-off, a civil debate with moments of modest policy clashes and few instances of Democrat-on-Democrat confrontation.

Immigration was also on their minds as the candidates’ minds as they pointed to the searing photos of a drowned Salvadoran father and his toddler daughter at the Rio Grande and blamed Mr Trump and his policies on migrants crossing into America illegally.

Former Obama administration housing chief Julian Castro said: “Watching that image of Oscar and his daughter Valeria was heartbreaking. It should also piss us all off.”

Ms Warren spent the evening at centre stage, a top-tier candidate whose campaign has gained ground in recent weeks as she has released a near-constant stream of policy proposals.

She was flanked by lower-tier candidates including Senator Cory Booker of New Jersey, former Texas Representative Beto O’Rourke and Senator Amy Klobuchar of Minnesota, who needed big moments to help spark momentum in the crowded field.

Election 2020 Debate
Democratic presidential candidate Beto O’Rourke (Wilfredo Lee/AP)

Several candidates, including Mr Castro, New York Mayor Bill de Blasio, Representative Tim Ryan of Ohio and former Representative John Delaney of Maryland were eager to jab their rivals on issues including health care and immigration.

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

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

Mr De Blasio, who joined Ms Warren in raising his hand on health insurance, cast the debate as part of “the battle for the heart and soul of our party”.

Mr Trump, the elephant not in the room, was in the air travelling 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!”

Mr Trump also tweeted that the NBC News network “should be ashamed” after the coverage was marred by a technical glitch.

NBC’s technical problem came following a half-time switch from Lester Holt, Savannah Guthrie and Jose Diaz-Balart — who were the primary questioners in the first hour — to Chuck Todd and Rachel Maddow.

Mr Todd was interrupted by voices as he tried to direct a gun control question to Ms Warren.

It caused a brief, uncomfortable period of confusion before Mr Todd switched to a lengthy, unplanned commercial break.

Following the break, the issue was corrected.

But it gave Mr Trump an opening to attack one of his favourite targets, the news media.

NBC and MSNBC “should be ashamed of themselves for having such a horrible technical breakdown in the middle of the debate,” he said. He called it “truly unprofessional.”

The Republican president helped unite the Democrats, but the candidates did not agree on everything.

Mr Castro assailed fellow Texan Mr O’Rourke for not calling for fully decriminalising crossing the US-Mexico border illegally.

“I just think it’s a mistake, Beto,” he said, adding that Mr O’Rourke would agree with him “if you did your homework on this issue”.

Mr O’Rourke said he does not support fully decriminalising such border crossings because of fears about smugglers of drugs and people.

New Jersey Senator Mr Booker also sided with Mr Castro, arguing for full decriminalisation.