The erosion has been fairly broad, spanning virtually all demographic groups. But in a longer-term context, the president’s weakness is most stark in one respect: his deficit among women.

Women were supposed to carry the first female major-party nominee to victory four years ago, as many assumed that Mr. Trump’s treatment of women, including allegations of sexual assault, would prove to be his undoing. But women might be his undoing this time. He trails Mr. Biden by 25 points among them, far worse than his 14-point deficit four years ago. He still leads among men by six points in the most recent polls, about the same margin as he led by in the final polls of registered voters in 2016.

Over the shorter term, the decline in the president’s standing has been particularly pronounced among white voters without a college degree, helping to explain why the Trump campaign has felt compelled to air advertisements in Ohio and Iowa, two mostly white working-class battleground states where Mr. Trump won by nearly 10 points four years ago.

In the most recent polls, white voters without a college degree back the president by 21 points, down from 31 points in March and April and down from the 29-point lead Mr. Trump held in the final polls of registered voters in 2016.