Donald Trump's campaign mocked the debate as "the best argument" for his re-election.(file)

Ten Democrats clashed yesterday in the first debate of the 2020 presidential race, with Elizabeth Warren cementing her status as a top-tier candidate and several underdogs using the issue of immigration to clamor for the limelight.

The biggest American political debate since the 2016 presidential campaign is occurring over two nights in Miami, climaxing Thursday with former vice president Joe Biden squaring off against number two candidate Bernie Sanders and eight others.

Yesterday's first take was a spirited encounter among ex-congressman Beto O'Rourke, Senator Cory Booker, former San Antonio mayor Julian Castro and New York Mayor Bill de Blasio on everything from health care and economic inequality to climate action, gun violence, Iran and immigration.

They hit out at President Donald Trump's economic and immigration policies, but diverged on how zealously the next president should shift the country onto a more liberal course.

Despite heavy campaigning in early voting states, millions of Americans were tuning in to the 2020 race for the first time.

What they heard from the start was the progressive ideology of Warren, the night's only candidate polling in double digits, and her knocking a rigged system.

"When you've got a government, when you have an economy that does great for those with money and is not doing great for everyone else, that is corruption, pure and simple," she said. "We need to call it out."

On such a crowded stage the hopefuls had limited time to impress, and of the low-polling candidates, Castro, De Blasio and Amy Klobuchar seemed to make the most of the opportunity.

The debate's backdrop -- the mushrooming border crisis, the detention of migrant children in squalid conditions and a shocking photograph of a Salvadoran man and his daughter drowned in the Rio Grande -- prompted tense exchanges.

Castro, the only Latino in the race, who wants to decriminalize unauthorized immigration as part of a sweeping immigration reform, called the photograph "heartbreaking."

"It should also piss us all off," he said, "and it should spur us to action."

O'Rourke, Booker and later Castro notably slipped into Spanish as they spoke about the migration crisis, openly courting an increasingly influential voting bloc.

De Blasio earned loud applause when he addressed citizens who have been told that immigrants have helped cause their woes.

"The immigrants didn't do that to you!" De Blasio boomed. "The big corporations did that to you."

Klobuchar also rushed to the defense of immigrants, saying "they are America."

The Minnesota senator, who occupies the party's more centrist lane, confidently pushed back as a male rival claimed he was the lone candidate to pass legislation protecting women's reproductive rights.

"There's three women up here who have fought pretty hard for a woman's right to choose," Klobuchar said.

Trump a target

More than any other candidate, Warren, 70, has given a picture of her presidential priorities, like instituting a wealth tax, breaking up big tech companies and securing the US election system.

In closing remarks she recalled growing up in Oklahoma where a government-funded community college helped her catch a break.

"I am in this fight because I believe that we can make our government, we can make our economy, we can make our country work not just for those at the top," she said.

Candidates were divided over a controversial question for the party: whether to abolish private insurance and switch to government health care.

De Blasio confronted O'Rourke over his reluctance to phase out private insurance despite it "not working for tens of millions of Americans."

While there was sniping among debaters, some trained their anger on Trump. Many candidates have said he should face impeachment proceedings.

Congresswoman Tulsi Gabbard savaged him on foreign policy, saying "this president and his chickenhawk cabinet have led us to the brink of war with Iran."

Despite flying to Asia for the G20 summit Wednesday, Trump tuned in from Air Force One and did not miss the chance to knock his would-be opponents via Twitter.

His verdict as the evening progressed: "BORING!"

Meanwhile Trump's campaign mocked the debate as "the best argument" for his re-election.

"The Democrats proposed a radical government takeover of American society that would demolish the American dream," it said in a statement.

Many voters will be watching more closely Thursday and the clash of the old guard featuring Biden, 76, and Sanders, 77.

The pair will face up-and-comers like Senator Kamala Harris, the only black woman in the race, and Pete Buttigieg, the 37-year-old gay mayor of South Bend, Indiana.