After Rebuking Trump, Democratic Candidates Attack One Another

“I very much agree with Senator Warren on raising more tax revenue from millionaires and billionaires,” Mr. Buttigieg said. “I just don’t agree on the part about spending it on millionaires and billionaires when it comes to their college tuition.”

Ms. Warren, who supports free tuition at public colleges and canceling most student debt, was ready with a quick rejoinder: “The mayor wants billionaires to pay one tuition for their own kids. I want a billionaire to pay enough to cover tuition for all of our kids.”

Fault lines also emerged quickly on matters of the economy, with two candidates — Ms. Klobuchar and Senator Bernie Sanders of Vermont — diverging on the merits of Mr. Trump’s new trade deal with Mexico and Canada, which the House approved only hours earlier. And in the first stage of the debate, there were hints of friction over proposals by Mr. Sanders and Senator Elizabeth Warren to create broad new college tuition benefits and to impose new taxes on the country’s largest private fortunes.

The debate at Loyola Marymount University here in Los Angeles seemed to inaugurate a new phase in the campaign, with only seven candidates onstage and a deeper airing of substantive differences on the issues. Four top-tier candidates remain in the race, with former Vice President Joseph R. Biden Jr. leading in the national polls, followed by Mr. Sanders and Ms. Warren, and Mr. Buttigieg, surging in the earliest primary and caucus states. But less than two months before Iowa, the race remains highly fluid, with considerable remove for movement not just among the top few candidates but among the underdogs as well.

Mr. Trump provided a backdrop for the forum, and not only because of his newly embattled status and his anticipated victory on trade. In a series of richly substantive exchanges, on issues such as climate change, press freedom and American relations with China and Israel, the candidates held him up as the embodiment of all they would not do with the presidency.

The Democrats were particularly unsparing with regard to the president’s foreign policy record, calling him an ally to tyrants and a figure of fun on the international stage. Ms. Klobuchar alluded to Mr. Trump’s tempestuous departure from a recent NATO summit after a video surfaced of several foreign leaders joking about him. “He is so thin-skinned that he walked, he quit,” she said, adding, “America doesn’t quit.”

Several of the leading candidates vowed to take a more coordinated and forceful approach to dealing with China, including on human rights. Mr. Biden said he would seek to levy United Nations sanctions against the Chinese government for rounding up Muslim Uighurs in camps, while Mr. Buttigieg said he was open to the possibility of boycotting the 2022 Olympics in Beijing.