But interviews with dozens of people who voted for Mr. Trump reveal a more fluid picture. Some were die-hard supporters who were hungry for any information to support Mr. Trump’s claims — against all evidence — that he won the election. For these voters, no data could convince them otherwise.

Others were more uncertain. Nearly all of the people interviewed said they believed at least some fraud had been perpetrated, but whether that added up to Mr. Trump’s being the true winner was much harder to know.

The reasons for doubting the outcome were many. Misinformation played a role. So did signaling by Republican leaders, first among them Mr. Trump.

Partisanship was powerful, too: Some were so distrustful of Democrats that they were open to arguments about fraud in large part because Democrats were not. Still others said election fraud was simply not that unusual a phenomenon. And in a sign of how much Americans of both parties are living in political bubbles, many expressed surprise that Mr. Biden could have won, given that they knew no one who voted for him.