One theory gaining new attention was that renewed efforts this year to draw in more rural voters and those without four-year college degrees—groups supportive of Mr. Trump—were helpful but insufficient.

Some have come to believe that a distrust of institutions is more pervasive than anticipated across many voter groups, and that it leads conservative voters, even those with college degrees and urban addresses, to avoid participating in polls in disproportionate numbers. If so, the problem likely can’t be corrected by adding more members of any one demographic group to a polling sample, they said.

“I readily admit that there were problems this year, but it is too soon to know the extent of the problems, or what caused them,’’ said Courtney Kennedy, who supervises poll methodology for the Pew Research Center. In both 2016 and 2020, she said, ”there was a widespread overstatement of Democratic support,’’ but the causes this time around could be different.

Some pollsters said that state and local polls had been particularly off-course. “District-level polling has rarely led us—or the parties and groups investing in House races—so astray,’’ wrote analyst David Wasserman of the Cook Political Report, whose review of private polling by political groups had led him to predict Democratic gains in the House of between 10 and 15 seats, rather than the GOP gains that now seem likely.