A depressed and demoralized Democratic Party is beginning the painful slog into a largely powerless future, as its leaders grapple with how deeply they underestimated Donald J. Trump’s resurgent hold on the nation.

The nationwide repudiation of the party stunned many Democrats who had expressed a “nauseous” confidence about their chances in the final weeks of the race. As they sifted through the wreckage of their defeats, they found no easy answers as to why voters so decisively rejected their candidates.

In more than two dozen interviews, lawmakers, strategists and officials offered a litany of explanations for Vice President Kamala Harris’s failure — and just about all of them fit neatly into their preconceived notions of how to win in politics.

The quiet criticism, on phone calls, in group chats and during morose team meetings, was a behind-the-scenes preview of the intraparty battle to come, with Democrats quickly falling into the ideological rifts that have defined their party for much of the Trump era.

What was indisputable was how badly Democrats did. They lost the White House, surrendered control of the Senate and appeared headed to defeat in the House. They performed worse than four years ago in cities and suburbs, rural towns and college towns. An early New York Times analysis of the results found the vast majority of the nation’s more than 3,100 counties swinging rightward since President Biden won in 2020.

The results showed that the Harris campaign, and Democrats more broadly, had failed to find an effective message against Mr. Trump and his down-ballot allies or to address voters’ unhappiness about the direction of the nation under Mr. Biden. The issues the party chose to emphasize — abortion rights and the protection of democracy — did not resonate as much as the economy and immigration, which Americans often highlighted as among their most pressing concerns.