In the aftermath of Thursday’s presidential debate, as Jill Biden led President Biden off the stage, former Senator Claire McCaskill, the Missouri Democrat, raised what she called a “hard and heartbreaking” question.

“You have to ask,’’ she said on MSNBC, “how did we get here?”

Barely seven weeks before Democrats gather in Chicago to formally nominate Mr. Biden for a second term, the Democratic Party is in crisis. Many party leaders, donors, activists and ordinary voters, stunned by the president’s faltering debate appearance, now fear he will lose to former President Donald J. Trump and drag Democrats to devastating defeats in congressional and state elections.

The answer to Ms. McCaskill’s question is a complicated mix of historical circumstance and structural deficiencies, a party struggling with ideological and generational fissures, and an aging Democratic president who spent his life battling for this job.

Mr. Biden is surrounded by a tight circle of longtime aides and family members who have encouraged his desire to seek a second term. But interviews with top party strategists, office holders and people close to Democrats seen as possible presidential hopefuls suggest that, just as crucially, party leaders were lulled into complacency or pressed to step in line at crucial moments when they might have persuaded Mr. Biden to step aside.

Many of them, including the president’s top aides, drew what could prove to be overly encouraging lessons from Mr. Biden’s victory against Mr. Trump in 2020, his run of policy victories as president and the party’s surprisingly strong showing in the midterm elections of 2022.

“It was the ’22 elections,” said David Plouffe, who was the senior adviser to President Barack Obama’s re-election campaign in 2012. “We’ve had three good elections in a row. The feeling was, ‘Let’s stay the course.’”

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