“This is not the Republican Party,” he added, citing his relationships with “my Republican friends in the House and Senate.”

There is no disagreement among Democrats about the urgency of defeating President Trump. But Mr. Biden’s singular focus on the president as the source of the nation’s ills, while extending an olive branch to Republicans, has exposed a significant fault line in the Democratic primary.

Democrats, like Senators Bernie Sanders and Elizabeth Warren, see the president as a symptom of something deeper, both in a Republican Party overtaken by Trumpism and a nation cleaved by partisanship. Simply ousting Mr. Trump, they tell voters, is not enough…

“I feel like the party went through this and the 2016 election showed that Trumpism isn’t just Donald Trump — it’s the entire Republican Congress, too,” said Rebecca Katz, a progressive Democratic strategist unaligned in the 2020 contest. “Until there is someone in the Republican Party who can stand up to Trump, then none of them are better than Trump.”