Democratic presidential candidates from left, Sen. Elizabeth Warren, D-Mass., former Vice President Joe Biden, Sen. Bernie Sanders, I-Vt., Sen. Amy Klobuchar, D-Minn., and businessman Tom Steyer participate during a Democratic presidential primary debate Thursday, Dec. 19, 2019, in Los Angeles. Credit: Chris Carlson | AP

LOS ANGELES — A pointed and personal clash over the corrupting influence of wealthy campaign donors dominated the Democratic presidential debate Thursday, as Sen. Elizabeth Warren and South Bend, Indiana, Mayor Pete Buttigieg tangled over each other’s ability to govern with integrity.

The confrontation, which ignited after weeks of simmering disagreements between the two, provided the biggest fireworks in a night filled with elevated voices, waving arms and some of the most aggressive exchanges of any of the debates this year.

Seven candidates onstage — the smallest debate of the year — fought over health care policy, age and experience, and whether they had the ability to defeat President Donald Trump.

Amid the jabs and counterpunches, former vice president Joe Biden focused on his central campaign message in his smoothest and most commanding debate of the year. He said he would fight to regain a spirit of cooperation with Republican lawmakers, despite a poisonous political atmosphere in Washington.

“If anyone has reason to be angry with the Republicans and not want to cooperate, it’s me the way they’ve attacked me and my son and my family,” he said, alluding to the recent Republican focus on his work in Ukraine when he was vice president and his son Hunter Biden’s work with a Ukrainian gas company. “But the fact is, we have to, we have to be able to get things done.”

Warren’s assault on Buttigieg for holding fundraisers with wealthy donors, including a recent event in Napa, California, which took place in a catered wine cave with a crystal chandelier, marked a dramatic shift in her past practice of avoiding debate-stage conflict with her rivals.

“Think about who comes to that,” the senator from Massachusetts said, making the case that big-dollar donors don’t have the same concerns as those who are struggling with student loan debt or medical debt. “Billionaires in wine caves should not pick the next president of the United States.”

Buttigieg, who has recently pulled ahead of Warren in Iowa polling, hit back by arguing he had the lowest net worth of anyone onstage. He accused Warren of “issuing purity tests you cannot yourself pass,” since she held closed-door fundraisers during her 2018 Senate campaign and transferred $10.4 million of that money into her presidential account.

Sen. Bernie Sanders of Vermont, who like Warren does not hold high-dollar events in the current campaign, targeted Biden for a fundraising strategy similar to Buttigieg’s. “He’s received contributions from 44 billionaires,” Sanders said. “Pete, on the other hand, is trailing. … You only got 39 billionaires contributing.”

Sen. Amy Klobuchar of Minnesota, who separately targeted Buttigieg for having a thin political résumé, quickly joined in. “I did not come here to listen to this argument!” said Klobuchar, who made her most forceful debate appearance. She added: “I have never even been to a wine cave.”

The stark differences in style and substance, coming just 45 days before the first votes in the Iowa caucuses, gave a still-undecided party some clear choices between moderation and disruption, healing political divides and stoking partisan fights and fresh energy and deep political experience.

Buttigieg was the target of the most attacks, with Sanders dismissively describing him as an “energetic guy” and Klobuchar referring to the 37-year-old mayor as a “local official” who has played down the more substantial political experience of others on the stage.

“I just think you should respect our experience when you look at how you evaluate someone who can get things done,” she said.

Buttigieg responded by arguing that his accomplishment was not the size of his electoral win in his city but the place in which he won. “Want to talk about the capacity to win?” he said. “Try putting together a coalition to bring you back to office with 80 percent of the vote as a gay dude in Mike Pence’s Indiana.”

Klobuchar, who has spent much of the campaign imploring Democrats to choose someone like her who has won in Trump-favoring areas, shot back by pointing out that Buttigieg had lost a state treasurer’s race in 2010 by more than 20 points, the only time he ran statewide.

Biden distanced himself from the more venomous exchanges onstage, but he was at the center of one of the debate’s more emotional moments when he demonstrated the stutters of people who approach him at rallies to tell him their struggles with the condition, which Biden had when he was younger.

Afterward, former White House press secretary Sarah Sanders tweeted, “I I I I I I I I I I I I I I I hhhave absolutely no idea what Biden is talking about,” allowing Biden to bite back online after the event.

“It’s called empathy. Look it up,” Biden tweeted to Sanders. Sanders responded by claiming that she was not aware of his stutter. “I apologize and should have made my point respectfully,” she wrote.

Warren entered the sixth debate struggling to regain the popularity she enjoyed in the fall. After months of making fighting corruption and the interests of political donors a pillar of her campaign, she went back to that theme in her responses to questions about impeachment and how she would persuade voters who felt the economy had improved under Trump.

“I’m proud to stand on a stage with people who see that America’s middle class is being hollowed out and the working families and poor people are being left behind,” Warren said. “What we need to talk about, though, is why that has happened. And the answer is we’ve got a government that works great for those with money and doesn’t work for much of anyone else.”

Buttigieg argued for a more moderate path that could nonetheless rally the country.

“We’ve got to break out of the Washington mind-set that measures the business of an idea by how many trillions of dollars it adds to the budget or the boldness of an idea by how many fellow Americans it can antagonize,” he said.

The Democratic debate came at a time of significant political tumult, for both the country and the party. Just a day before candidates walked onstage, House Democrats voted to impeach Trump, and on the day of the debate the same body passed a new bipartisan North American trade deal that has been a top legislative priority of the White House.

From the opening answers, the party’s differences were on display, with Sanders saying he will oppose the new trade deal passed by the House, while Klobuchar praised the deal’s labor and environmental standards.

“I will not be voting for this agreement, although it makes some modest improvements,” Sanders said, after calling for further improvements, including direct language to address the threat of climate change.

After nearly a year of campaigning, the nomination fight remains fractured, without a front-runner who can claim a clear path to acquiring a majority of delegates before the convention. Biden, the longtime polling leader, boasts the support of one in four Democratic voters nationally but finds himself in a statistical four-way tie in the first two nominating states of Iowa and New Hampshire, with about 18 percent of the votes, according to Washington Post polling averages.

The biggest change from the first debate this summer has been the dramatic winnowing of the field. Onetime political stars such as former congressman Beto O’Rourke of Texas and Sen. Kamala Harris of California have left the race, as have three current or former governors. Out of an initial field of more than two dozen, 15 are still running, but only seven were able to meet the 4 percent polling threshold required along with a specific number of donors to make the debate.

Of that group, six were white, five were men, three were septuagenarians and none were African-American or Latino. Businessman Andrew Yang, whose parents emigrated from Taiwan, was the only racial minority onstage in a state where nonwhites are propelling the Democratic Party.

“It’s both an honor and disappointment to be the only candidate of color on the stage,” Yang said, before mentioning Harris and Sen. Cory Booker of New Jersey, who did not make the cut for the debate. “I miss Kamala, I miss Cory — though I think Cory will be back.”

Yang pointed out that households of color often don’t have the disposable income to donate to political campaigns, and touted his plan for a universal basic income.

“I guarantee if we had a freedom dividend of $1,000 a month, I would not be the only candidate of color on this stage tonight,” Yang said.

The candidates largely agreed on the need to combat climate change, with Biden and Sanders both saying that they would sacrifice jobs that benefit from oil and natural gas production.

“The issue now is whether to save the planet for our children and grandchildren,” Sanders said.

Billionaire investor Tom Steyer, who has been running on a platform of combating climate change and getting money out of politics, did make the debate stage. Fielding the first question of the night, on Trump’s impeachment, Steyer noted he had called for impeachment more than two years ago.

“If we want the American people to understand what’s going on, we need to have the administration officials testify on TV,” he said about the coming Senate trail.

The debate included several stretches on foreign policy, with candidates largely agreeing on being tougher on China, encouraging a two-state solution in Israel, and renewing efforts to close the U.S. military prison in Guantánamo Bay. Biden, who is often reluctant to show any fissures with former President Barack Obama, criticized the administration’s decision to add troops in Afghanistan.

“I’m the guy from the beginning who argued that it was a big, big mistake to surge forces to Afghanistan, period,” he said. “We should not have done it..”

The candidates who have polled at the top are in their 70s, and they were pressed over comments President Obama recently made attributing most of the world’s problems to “old people, usually old men, not getting out of the way.” As Sanders was being asked about his age, he interjected, “I’m white as well!” before saying he disagreed with the former president.

“I’m going to guess he wasn’t talking about me,” Biden said. “I’m running because I’ve been around. On my experience — because with experience, hopefully comes judgment, and a little bit of wisdom.”

When Warren was told she would be the oldest president ever inaugurated, she responded: “I’d also be the youngest woman ever inaugurated.”