LOS ANGELES (NYTIMES) - The Democratic presidential candidates voiced strong support for the impeachment of United States President Donald Trump in the sixth primary debate on Thursday (Dec 19) night, but all of them seemed to accept the likelihood that he would remain in office after a looming trial in the Republican-controlled Senate and endure as their rival in the general election.

For the second consecutive debate, the top Democratic contenders began by training their fire on Mr Trump rather than on one another, describing him as an out-of-control president with little regard for the norms of his office or the rule of law.

But as the debate neared its halfway point, the tensions that have been building for weeks between Senator Elizabeth Warren of Massachusetts and Mayor Pete Buttigieg of South Bend, Indiana, over campaign funding and transparency reached a boiling point, playing out in a strikingly sharp and at times personal exchange.

"So the mayor just recently had a fund raiser that was held in a wine cave full of crystals," Ms Warren said, adding that "billionaires in wine caves should not pick the next president of the United States".

Mr Buttigieg protested: "You know, according to Forbes magazine, I'm literally the only person on this stage who is not a millionaire or a billionaire. So, this is important. This is the problem with issuing purity tests you cannot yourself pass.

"Senator," he added, "Your net worth is 100 times mine."

"I do not sell access to my time," Ms Warren rebuked him.

Their exchange was curtailed by Senator Amy Klobuchar of Minnesota. Sensing an opening to cast herself as above the fray and focused on party unity, she jumped in with some humour.

"I did not come here to listen to this argument," she said. "I came here to make a case for progress. And I have never even been to a wine cave. I have been to the wind cave in South Dakota."

The ideological clash between Mr Buttigieg and Ms Warren continued over which Americans should qualify for free college. Mr Buttigieg said: "If you're in that lucky top 10 per cent - I still wish you well, don't get me wrong - I just want you to go ahead and pay your own tuition.

"I very much agree with Senator Warren on raising more tax revenue from millionaires and billionaires," he 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 cancelling 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 Ms 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 in Los Angeles seemed to inaugurate a new phase in the campaign, with only seven candidates on stage and a deeper airing of substantive differences on the issues.

Four top-tier candidates remain in the race, with former vice-president Joe Biden 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 room 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 US 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.

"We're not looking for a war," Mr Biden said, "but we've got to make clear: We are a Pacific power and we are not going to walk away."

If the candidates were united in heaping opprobrium on Mr Trump a day after he was impeached by the House, few offered fresh arguments in favour of impeachment. Prodded by a moderator to explain how they would persuade a larger share of the country to support removing the President from office, the leading candidates mainly pivoted to the themes of their stump speeches.

Mr Biden said it was high time to "restore the integrity of the presidency", while Mr Sanders castigated Mr Trump for having "sold out the working families of this country" and Ms Warren branded him "the most corrupt president in living history".

The candidates, explicitly or by implication, all suggested they would be best equipped to make that case against him next autumn.

In each case, however, the candidates seemed to anticipate a general election battle with Mr Trump as a foregone conclusion, evincing little optimism that he could be removed from office in the impeachment process.

Mr Buttigieg nudged voters' attention to the general election, arguing, "No matter what happens in the Senate, it is up to us in 2020."

A lonely voice of scepticism came from Mr Andrew Yang, the former tech executive mounting an underdog campaign, who described impeachment as a distraction from more important economic issues.

Suggesting Mr Trump's acquittal in the Senate was a foregone conclusion, Mr Yang likened it to "a ballgame where you know what the score is going to be". Democrats, he said, should focus instead on offering a "new positive vision for the country".

Economic and trade issues took on a more prominent role in the discussion. Just minutes into the debate, a question about trade highlighted tension.

Mr Sanders said that he would not support the trade agreement that the House passed on Thursday, calling it a "modest improvement over what we have right now" but arguing that it would not do enough to support workers and farmers. He also appeared to take an oblique swipe at Mr Biden's past support of measures like the North American Free Trade Agreement.

Ms Klobuchar, a moderate, immediately seized on a chance to draw a contrast with Mr Sanders, saying she would support the new trade agreement, which she called a "much better deal" for "farmers in the Midwest" and others who have struggled under strained international trade relations.

They did not clash directly in the first minutes of the debate, but Ms Warren and Mr Buttigieg quickly sketched out two sharply divergent approaches to improving the economy and addressing inequality.

"Oh, they're just wrong!" Ms Warren bristled when asked about concerns regarding her tax plans, going on to defend her wealth tax proposal, which would affect the richest Americans to help pay for significantly expanding the social safety net.

Soon after, Mr Buttigieg took a thinly veiled swipe at that approach, emphasising the importance of "promises that we can keep without the kind of taxation that economists tell us could hurt the economy".

"We've got to break out of the Washington mindset that measures the bigness of an idea by the trillions of dollars it adds to the budget or the boldness of an idea by how many Americans it can antagonise," he said.

In a pointed answer early in the evening, Mr Yang lamented the racial homogeneity of the rest of the debaters onstage. When a moderator noted that Mr Yang was the only member of a minority group among the candidates, Mr Yang described that distinction as "both an honour and disappointment". He suggested that the economic distress of black and Latino voters made it harder for them to thrive in politics or to donate to political candidates.