A large number of voters — women, young people and those who are Black or Latino — have all soured on Mr. Biden’s performance, according to polls conducted since the withdrawal of American troops from Afghanistan, even while Mr. Biden has retained more of his support among men, college-educated white voters and older voters.

The shifts could be temporary. Perhaps the enactment of the president’s stalled legislative initiatives in Congress would be enough for the president to renew his reputation with Democratic-leaning voters. But for now, the differences in attitudes about Mr. Biden between men and women, young and old, Hispanic and non-Hispanic and perhaps even white and Black have grown unusually small. The pattern defies the decades-long tendency for Democrats to fare better among women than men, and among the young than the old…

While most major demographic divides have eroded recently, one has continued to widen: the education gap among white voters.

Mr. Biden’s support has held up among white college graduates, according to the recent polls, while his standing among white voters without a degree has dropped to 30 percent, down from 37 percent in polls taken in June and July.

Even if Mr. Biden could rejuvenate his support among traditional Democratic constituencies, weakness among white voters without a degree would still be a serious challenge for Democrats heading into the midterm election next year, when the president’s party usually suffers a drubbing.