Elizabeth Warren and Pete Buttigieg have clashed over who is funding their presidential campaigns, with the South Bend mayor accusing his rival of issuing “purity tests” she could not match, and the Massachusetts senator attacking him for holding events in caves bedecked with “$900 bottles of wine”.

In exchanges between the two candidates who have challenged each other for frontrunner status and who represent different ideological strands of the party, Ms Warren laid into Mr Buttigieg for holding closed-door fund-raising events where he spent time “with millionaires or billionaires”.

Referring to a now-infamous fundraising event Mr Buttigieg held at a winery in California’s Napa Valley, Ms Warren asked voters to consider who attends events where such expensive wine is served.

“We made the decision many years ago that rich people in smoke-filled rooms would not pick the next president of the United States,” she said.

“Billionaires in wine caves should not pick the next president of the United States.”

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

Mr Buttigieg was quick to fire back, pointing to a Forbes magazine article that said he was the only individual among the seven on stage who was not a “millionaire or a billionaire”.

“So, this is important. This is the problem with issuing purity tests you cannot yourself pass,” he said. “Senator, your net worth is 100 times mine. Suppose you went home and felt the holiday spirit – I know this isn’t likely, but stay with me – and decided to go on PeteButtigieg.com and gave the maximum donation allowable by law, would that pollute my campaign because it came from a wealthy person? No. I would be glad to have that support.”

The back-and-forth between the two marked one of the few tense moments of the night. That may have reflected the fact that the debate in Los Angeles, the sixth the party has held as it seeks to select a presidential candidate, was held just a day after Donald Trump was impeached by the House of Representatives.

The issue of impeachment was quickly covered with all seven candidates – also on stage were Joe Biden, Bernie Sanders, Amy Klobahcar, Andrew Yang and Tom Steyer – saying they applauded the move.

Yet Mr Yang, the only candidate of colour participating in the debate – indeed, there was more diversity among the journalistic questioners than the candidates – warned that Democrats may be overthinking that issue, compared to most Americans.

Donald Trump announces that Jeff Van Drew will be joining the Republican Party

“We have to stop being obsessed over impeachment, which unfortunately strikes many Americans like a ball game where you know what the score is going to be, and start actually digging in and solving the problems that got Donald Trump elected in the first place,” he said.

Ahead of Thursday’s debate at Loyola Marymount University, a CNN poll placed Mr Biden at the front of the pack on 26 points, followed by Mr Sanders on 20, Ms Warren on 16, Mr Buttigieg on 8, and latecomer Michael Bloomberg on 5.

In Iowa, the first state to vote and which holds its primary on 3 February, Ms Klobuchar has steadily improved her standing, moving into what she has termed a “solid fifth”, as she has protected herself as a compromise candidate lodged somewhere between the party’s two edges.

To date, her debates performances have not been remarkable. On Thursday, with fewer people on the stage, she took the opportunity to secure some headlines by again attacking Mr Buttigieg over his lack of experience, and for criticising the other candidates’ work in Washington.

“The point is, we should have someone heading up this ticket that has actually won and been able to show that they’ve gathered the support that you talk about of moderate Republicans and independents,” said the Minnesota senator. “I think a track record of getting things done matters.”

Mr Buttigieg, who found himself under attack from several quarters, jabbed back.

“If you want to talk about the capacity to win, try putting together a coalition to bring you back to office with 80 per cent of the vote as a gay dude in Mike Pence’s Indiana,” he said.

Mr Biden and Mr Sanders had steady evenings, with the former vice president not making any major slips, something that has marred some of his previous performances.

Asked about recent comments by Barack Obama that there would be less problems in the world if there were more women leaders and fewer “old men”, he responded: “I’m going to guess he wasn’t talking about me either.”

Yet, Mr Biden again dodged a question about whether he was prepared to commit to serving two terms if he won, something that would take him into his 80s. “I’m not even elected one term yet, let’s see where we are. Let’s see what happens.”

When it was pointed to Ms Warren she would also be the “oldest president ever inaugurated” if elected, she had a very sharp response. “I’d also be the youngest woman ever inaugurated.”

Billionaire Mr Steyer, who has been criticised for spending millions of dollars of his own money to gain enough name recognition to qualify for the debate, had his strongest moment when talking about climate change.

He said focusing on clean energy development could be a major driver of economic growth. “This is our greatest opportunity to reinvent this country,” he said.

The climate change crisis is an issue on which Mr Sanders has also been outspoken. He was so again in Los Angeles.

“Just maybe, instead of spending $1.8 trillion a year globally on weapons of destruction, maybe an American president – i.e Bernie Sanders – can lead the world,” he said of himself.