Mayor Pete Buttigieg finally got the top-tier treatment during the sixth Democratic presidential debate hosted in Los Angeles on Thursday, and Sen. Amy Klobuchar found the spotlight. The seven Democrats on stage, which also included former Vice President Joe Biden, Sen. Bernie Sanders, I-Vt., Sen. Elizabeth Warren, D-Mass., entrepreneur Andrew Yang and billionaire Tom Steyer, spent three hours at Loyola Marymount University in an at-times contentious debate that took place with less than two months before voters begin caucusing in Iowa, the first contest state. Biden remains the front-runner in national surveys, though Buttigieg has recently surged in the first contest states of Iowa and New Hampshire. Read more: 'Pete's wine cave': Why Buttigieg's rivals are slamming his Napa fundraiser Biden, who is 77, refused to commit to running for a second term if he won in 2020. "Let's see where we are, let's see what happens," he said. Buttigieg's rise made him a target — and Klobuchar took aim at his argument that he is the most electable contender because of his wins in South Bend.

"Mayor, if you had won in Indiana, that would be one thing. You tried and you lost by 20 points," Klobuchar said, referring to Buttigieg's failed 2010 bid for state treasurer. The debate came one day after the House of Representatives voted for Trump's impeachment, setting him up to be the third president in history to face a Senate trial over whether he will be removed from office. Despite the harsh sparring, the Democrats on stage united in taking on Trump. "We know what a gift it would be to the future and to the country for literally anybody up here to become president of the United States compared to what we've got," Buttigieg said at one point. Here are the debate's top moments.

Democrats sound off on Trump impeachment

Donald Trump Timothy A. Clary | AFP | Getty Images

The debate kicked off Thursday night with a question on why the majority of Americans do not support impeachment and how they can make the case to voters. Six out of seven candidates focused on what they described as Trump's corruption. "This president has made corruption originally his argument, that he would drain the swamp. And yet he came to Washington, broke that promise and has done everything he can for the wealthy and the well connected," Warren said. Biden complained foreign leaders are viewed more favorably than the president of the United States, adding, "We need to restore the integrity of the presidency, of the office of the presidency." Steyer shifted the conversation to the upcoming impeachment trial in the Senate saying the witnesses the White House has prohibited from testifying should be called up. Senate Minority Leader Chuck Schumer has demanded that Senate Majority Leader Mitch McConnell call administration officials including Acting Chief of staff Mick Mulvaney and former National Security Advisor John Bolton to the witness stand. "If we want the American people to understand what's going on, we need to have the administration officials testify on television so we can judge," Steyer said during the debate. "The American people deserve to see the truth of these administrative people under oath so we can make up our mind." Yang broke with his rivals, saying Democrats must stop focusing on Trump and try to explain how they will make life better for the average voter.

"The more we act like Donald Trump is the cause of all our problems, the more Americans lose trust that we can actually see what's going on in those communities and solve those problems," he said. "We have to stop being obsessed with impeachment, which unfortunately strikes many Americans as a ball game when you know what the score is going to be, and start digging in and solve all the problems that got Donald Trump elected in the first place."

Sanders and Klobuchar part ways on coming USMCA vote

Democratic presidential candidate Sen. Bernie Sanders (I-VT) reacts to Sen. Amy Klobuchar (D-MN) during the Democratic presidential primary debate at Loyola Marymount University on December 19, 2019 in Los Angeles, California. Justin Sullivan | Getty Images

Sanders and Klobuchar divided on the United States-Mexico-Canada Agreement that will soon come to a vote in the Senate after the House of Representatives approved the measure late Thursday. The trade deal, an update to NAFTA, earned the support of an overwhelming majority of Democrats in the House after a push for stronger labor protections delivered the support of the AFL-CIO, the country's largest federation of labor unions. Sanders said the trade deal was a "modest improvement" that would do nothing to stop outsourcing, and that he would not support it. "What we need is a trade policy that stands up for workers, stands up for farmers," Sanders said. "And by the way, the word climate change, to the best of my knowledge, is not mentioned in this new NAFTA agreement at all — which is an outrage — so no, I will not be voting for this agreement." Klobuchar channeled Sen. Sherrod Brown, a fellow Midwesterner, "who has voted against every trade agreement that has come in front of him, and he's voting for this, and I am too." "I think this agreement, while Senator Sanders is correct there are some issues with it, is much better than the one originally proposed," Klobuchar said. After the debate, a Warren surrogate said the senator has not come to a final decision on how she will vote on the deal. "The senator is still considering all the different options," said Nelini Stamp, National Organizing Director at the Working Families Party. "She is considering what labor has to say, what community organizations have to say."

Warren on critics who say her proposed tax hikes would stifle growth: 'They're just wrong'

Sen. Elizabeth Warren (D-MA) speaks during the Democratic presidential primary debate at Loyola Marymount University on December 19, 2019 in Los Angeles, California. Justin Sullivan | Getty Images

Warren, who has proposed a bigger tax hike on the wealthy than anyone else in the race, was asked to answer criticism from economists who say her plans would stifle growth and investment. "Oh — they're just wrong," Warren responded, prompting cheers and applause from the Los Angeles crowd. Warren's wealth tax proposal includes slapping a 2% tax on net worth between $50 million and $1 billion and a 6% tax on wealth over 10 figures. Buttigieg offered an indirect rebuke to Warren in his response. While taxes on the wealthy will have to go up, Buttigieg said, politicians should make "promises that we can keep" without "the kind of taxation that economists tell us could hurt the economy." Buttigieg and Warren have sparred in recent weeks over transparency. Buttigieg, polling behind Warren in national polls, recently surpassed her in polls in the early contest states of Iowa and New Hampshire.

Climate change takes center stage

Firefighters try to control a back burn as the Carr fire continues to spread towards the towns of Douglas City and Lewiston near Redding, California on July 31, 2018. Mark Ralston | AFP | Getty Images

The debate focused on climate change for what seemed like more time on Thursday than in earlier sessions. Moderator Tim Alberta, a chief political correspondent for Politico, asked whether candidates would support a federal program to subsidize the relocation of American families and businesses away from cities like Miami that scientists have said will be climate danger zones in decades to come. Sanders said the issue is not relocating people, but rather "whether we save the planet for our children and our grandchildren." "The issue, as you should know what the scientists are telling us, is that they have underestimated the threat and severity of climate change. You're talking about the Paris Agreement, that's fine. Ain't enough. We have got to – and I've introduced legislation to do this – declare a national emergency. The United States has got to lead the world," Sanders said. "What I think we need to do," said Klobuchar, is "get back into the international climate agreement. I will do that on day one." Klobuchar noted that California Gov. Gavin Newsom's efforts to implement climate solutions in the face of a spate of deadly fires in the state, have been "defied every step of the way by the Trump administration." Buttigieg said he would involve the average person in the movement rather than relying on politicians in Washington. "I've seen politicians in Washington saying the right thing about climate change as long as I've been alive. All these plans we have to get carbon neutral by 2050," he said. "Their impact is multiplied by zero unless something actually gets done."

Warren and Buttigieg spar over 'billionaires in wine caves'

Sen. Elizabeth Warren (D-MA) speaks as South Bend, Indiana Mayor Pete Buttigieg listens during the Democratic presidential primary debate at Loyola Marymount University on December 19, 2019 in Los Angeles, California. Justin Sullivan | Getty Images

Warren and Buttigieg delivered the big fight that everyone was waiting for following weeks of attacks between the two. After Warren criticized Buttigieg for a recent fundraiser hosted in a wine cave, Buttigieg accused Warren of issuing a "purity test" that she could not meet herself. "The mayor just recently had a fundraiser that was held in a wine cave full of crystals and served $900 a bottle wine. Think about who comes to that," Warren said, referring to an event Buttigieg held in Napa earlier this month. "We made the decision many years ago that rich people in smoke filled rooms should not pick the next president of the United States. Billionaires in wine caves should not pick the next president of the United States," she said. Buttigieg responded that he was "literally the only person on this stage who is not a millionaire or a billionaire." "Senator, your net worth is 100 times mine," he said. "We need the support from everybody who is committed to helping us defeat Donald Trump."

Tough talk on China

The candidates took a hard line on China's human rights track record, with some of the strongest language coming from Buttigieg. "If they do anything like Tiananmen Square...they will be isolated from the free world. And we will lead that isolation diplomatically and economically," Buttigieg said. The focus comes at a tense time in U.S.-China relations. China has rounded up its minority Uighur Muslim community into camps it says are designed to provide vocational training, but which critics describe as concentration camps. Biden said that the U.S. "should be moving 60% of our sea power to that part of the world to let, in fact, the Chinese understand that they're not going to get any further, we are going to be there to protect other folks." But the U.S. remains hugely reliant on China economically, both its manufacturing prowess and its consumers who buy U.S. products. Trump is in the midst of negotiating a trade deal with the country, with the terms of a "phase one" deal said to be agreed to, though details remain scant. Not everyone agreed with Buttigieg and Biden. Steyer took a more moderate stance. "We actually can't isolate ourselves from China. In fact, we have to work with them as a frenemy, people who disturb us, who we disagree with, but who in effect we are linked to in a world that is ever getting closer," he said.

Klobuchar bombards Buttigieg

Democratic presidential hopeful Minnesota Senator Amy Klobuchar speaks during the sixth Democratic primary debate of the 2020 presidential campaign season co-hosted by PBS NewsHour & Politico at Loyola Marymount University in Los Angeles, California on December 19, 2019. Frederic J. Brown | AFP | Getty images