Los Angeles (AFP) - Democratic White House candidates attacked President Donald Trump but also clashed sharply with one another during the Democratic debate, where a smaller field allowed lesser-known hopefuls to shine against the frontrunners.

One day after Trump's ignominious impeachment, seven candidates united on Thursday to declare the president must be defeated at the 2020 ballot box.

"We need to restore the integrity of the presidency," said frontrunner Joe Biden, who accused Trump of "dumbing down" the office "beyond what I even thought he would do."

Senator Bernie Sanders, who is second in the standings, blasted Trump as "running the most corrupt administration in the modern history of this country," 

Fellow Senator Elizabeth Warren said the president ignores the poor, to do "everything he can for the wealthy and the well-connected."

But after calls for greater moral leadership from the White House and crisp back-and-forth about trade policy, health care and how to lift more Americans out of poverty, the debate took a more aggressive tone with a series of heated clashes between candidates.

Warren launched her sharpest attacks yet on rival Pete Buttigieg, saying the 37-year-old mayor of South Bend, Indiana, has been holding closed-door fundraisers for the wealthy, including at a billionaire-owned "wine cave" in California.

"Your net worth is 100 times mine," Buttigieg, whose star has risen substantially in the past two months of campaigning, said.

"I don't sell access to my time," Warren, 70, shot back.

While Warren hit Buttigieg on money in politics, Senator Amy Klobuchar also sought to kneecap the mayor.

"I did not come here to listen to this argument. I came here to make a case for progress -- and I have never even been to a wine cave," she quipped.

Klobuchar was quick to remind viewers of her Midwestern roots as she called for "returning to sanity" in 2020. She also appeared to challenge the accomplishments of Buttigieg, a fellow Midwesterner, labeling him a "local official."

When Buttigieg suggested there were "bigger fish to fry," Klobuchar clapped back with a cutting retort: "I don't think we have bigger fish to fry than picking a president of the United States."

The final prime-time debate of 2019 featured just seven of the 15 Democrats still in the contest six weeks before the first nomination ballots are cast in Iowa in February.

The showdown marks a significant drop from the 10 candidates in November's debate, and the dozen who crowded the stage in October. 

Cozier quarters allowed for more extended exchanges about policy between participants hoping for their party's nomination to challenge Trump.

There was extra oxygen for lower-polling candidates including Klobuchar, billionaire activist Tom Steyer, and entrepreneur Andrew Yang, who sought to ease the exasperation many voters have expressed about American politics.

"We have to take every opportunity to present a new, positive vision for the country, a new way forward to help beat him in 2020," he said of Trump. 

"Because make no mistake, he'll be there at the ballot box."

In a statement calling the Democratic debate "drab" and "pessimistic," Trump's 2020 campaign said it was "even more clear why they felt they had to impeach President Trump. None of these characters has a chance."

Stutter trouble

Biden, popular with blue-collar workers and African Americans, has promised to raise America's standing back to the way it was under Barack Obama, rescuing it from the extreme polarization of Trump's tenure. 

The Democratic elder statesman has faced doubts about his health and age -- he is 77 -- and notably declined to pledge that he would run for a second term if he wins the White House.

"No, I'm not willing to commit one way or another," he said.

He appeared more feisty and less halting than in previous debates, interrupting moderators to slam the 78-year-old Sanders for a universal health care plan that could cost upwards of $30 trillion over 10 years.

But near the end of the 2.5 hour event, Biden committed an unforced error by imitating a stammering youth seeking advice on how to overcome a stutter, a problem Biden himself struggled with as a child.

The incident quickly resonated beyond the debate stage, with former White House spokeswoman Sarah Sanders mocking Biden on Twitter. 

Biden tweeted out damage control, saying "I've worked my whole life to overcome a stutter. And it's my great honor to mentor kids who have experienced the same."

Sarah Sanders apologized for her jibe.