Former Vice President Joseph R. Biden Jr. conjured the prospect of headlines like, “Look what happens when the Labour Party moves so, so far to the left.” Former Mayor Michael R. Bloomberg said that “Jeremy Corbyn’s catastrophic showing in the U.K. is a clear warning.” And Mayor Pete Buttigieg spoke of the need to “build a coalition and gather that majority.”
As Britain’s Labour Party grappled on Friday with its worst performance in more than 80 years, centrist Democrats across the Atlantic seized on those election results — with varying degrees of urgency — to argue that their own party risked losing in November by moving too far to the left. The day-after assessments threatened to deepen the tensions between moderates and progressives that have shaped the 2020 presidential campaign from the start.
The comparisons between the Labour Party and the Democratic Party, and between the candidates representing them, are far from perfect, and make it difficult to draw precise parallels to American political dynamics. The British election was focused in significant part on the issue of leaving the European Union, as Prime Minister Boris Johnson pressed to “get Brexit done” and convinced a solid majority of voters that he was on the right path. Mr. Corbyn faced controversies at home, including a crisis in his party over accusations of anti-Semitism.
But moderate presidential candidates, strategists and other party leaders in the United States said Friday that the results in Britain offered ominous signs about nominating a candidate perceived as out of the political mainstream.
“This is a warning shot that we shouldn’t repeat the mistakes made by Labour,” Rahm Emanuel, the former Chicago mayor, said in an interview.
James Carville, a longtime Democratic strategist who supports Senator Michael Bennet of Colorado for the presidency, was more explicit: “You can go so far left that you can lose to an unacceptable incumbent,” he said. “That’s the lesson. The lesson is screaming right in your face.”
Mr. Biden, a former Senate Foreign Relations Committee chairman and leading moderate contender, likely stands to gain the most, among the top contenders, from fears that the results in Britain offer warnings for Democrats. At a fund-raiser on Thursday, he appeared to echo aspects of Mr. Carville’s concerns, saying: “You’re also going to see people saying, ‘My God, Boris Johnson, who is kind of a physical and emotional clone of the president, is able to win.’”
Mr. Bloomberg, the former New York City mayor, joined in with a cautionary tweet. And at an event held by The Washington Post on Friday, Mr. Buttigieg, who studied at Oxford University as a Rhodes scholar, said “it’s a little tough to draw comparisons” between the British election and American politics because of the Brexit issue.
Yet asked the lesson “for the left,” Mr. Buttigieg, the mayor of South Bend, Ind., replied, “Well, it means that you’ve got to be ready to build a coalition and gather that majority.”
David Axelrod, President Barack Obama’s former chief strategist who also advised Britain’s Labour Party in 2015, called Brexit “a unique circumstance” and Mr. Corbyn “a uniquely weak candidate.’’
“But there’s no doubt that he also was further to the left than Britain wanted to go,” Mr. Axelrod added. “This is an election, a campaign. People are going to make those comparisons that they think are helpful to them, and do think a lot of Democrats are going to look at what happened there with some concern.”
Progressives took different lessons from the results, rejecting the idea that they were a harbinger of trouble for more liberal candidates like Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts. Adam Green, a co-founder of the Progressive Change Campaign Committee, said it was “a completely made-up narrative that there’s any similarity between this very unique U.K. election and the dynamics in this country.”
Waleed Shahid, the spokesman for Justice Democrats, a progressive organization that backed Representative Alexandria Ocasio-Cortez of New York — she shared a video from Labour on Twitter on Thursday — and other liberal Democrats in 2018, argued that Mr. Corbyn’s defeat was a different kind of warning sign: that Democrats must focus more on working-class candidates in 2020.
“The lesson for the entire Democratic Party coalition: Across the Western world, center-left parties are bleeding voters in postindustrial places to the right wing or to not voting at all,” Mr. Shahid said. “That’s a serious development that’s happening all over the Western world.”
Both Mr. Biden and Mr. Sanders are especially strong with working-class voters, polls show.
Still, Mr. Corbyn’s convincing defeat was a blow to members of the Sanders campaign who had expressed support for Labour. Claire Sandberg, a senior aide who is Mr. Sanders’s national organizing director, tweeted on Thursday afternoon, less than three hours before British polls closed, that the “Bernie team says #VoteLabour,” along with a photo of campaign staff members posing with red Corbyn signs.
A spokesman said Ms. Sandberg’s tweet was not the official position of the Sanders campaign. And aides and allies warned that the British and American elections are not apples-to-apples comparisons. There’s no American equivalent to the Brexit question, and Mr. Corbyn’s politics are far to the left of Mr. Sanders’s.
Thomas Wright, the director of the Center on the United States and Europe at the Brookings Institution, cautioned against directly equating Mr. Corbyn’s crushing defeat to the prospects of Mr. Sanders and Ms. Warren.
It would be a different situation, he said, if Mr. Sanders and Ms. Warren had publicly presented themselves as being in lock step with Mr. Corbyn.
“If they had said, ‘Corbyn represents the future and he’s fighting our fight in the U.K.,’ I think they would be in a fair bit of trouble today after that defeat,” Mr. Wright said. “But they sort of maintained some distance. So I think the effect of it will be somewhat limited.”
There is also little evidence so far that the results of a foreign election will affect how American Democratic voters think about the candidates, suggested John Anzalone, a Democratic pollster who works for Mr. Biden.
Still, he said of the results, “There’s plenty of people, beyond any of us, who make the argument that if you have too liberal, or an ultraliberal, Democratic nominee, it puts at risk what is at stake. Does it reinforce that? Absolutely, it reinforces that.”
Reid J. Epstein contributed reporting.