US Democrat Presidential candidate Bernie Sanders has defended his big-spending plans for government healthcare, but acknowledged some Americans would pay more tax to implement it.

On the second night of back-to-back Democratic debates, Mr Sanders said his Medicare-for-All healthcare plan would be fully paid for and reduce premiums for many but that some in the middle class might pay more.

“Yes, they will pay more in taxes but less in healthcare for what they get,” Mr Sanders said as the debate in Miami got started.

Democratic front-runner Joe Biden said he would repeal Republican President Donald Trump’s tax cuts for the wealthy.

“Donald Trump has put us in a horrible situation, we do have horrible income inequality,” Mr Biden said.

“I would be going about eliminating Donald Trump’s tax cuts for the wealthy.”

Mr Biden, who is making his third White House bid, and Sanders, a US senator who failed in his first bid in 2016, were at centre stage in Friday’s (AEST) debate.

The former vice-president has enjoyed a healthy lead in opinion polls since he entered the Democratic race to pick a challenger to Mr Trump in the November 2020 election.

Mr Sanders has been running second in most polls but has seen his support slipping in recent weeks, with some of his supporters switching to fellow progressive Elizabeth Warren.

The two top contenders shared the stage with eight other Democrats, including two other top-tier candidates: Pete Buttigieg, the mayor of South Bend, Indiana, and US Senator Kamala Harris of California.

The debate also included US Senators Michael Bennet and Kirsten Gillibrand, former Colorado Governor John Hickenlooper, US Representative Eric Swalwell, self-help guru Marianne Williamson and entrepreneur Andrew Yang.

All six are polling nationally around 1 per cent or less.

In a surprisingly heated first debate on Wednesday night, 10 Democratic contenders including Warren clashed over healthcare and border policy but agreed there was a desperate need to remove Mr Trump from the White House.

-AAP