Mr. Biden, however, is failing to generate widespread enthusiasm, the poll finds. For the past year he has been viewed more negatively than positively among the broader electorate—as has been the president—although Mr. Biden’s positive rating increased 5 percentage points between July and August to reach 39%.

“This poll is a warning for Democrats and the Biden team that there is still a lot of work to be done,” said Peter Hart, a Democratic pollster who worked on the survey with Bill McInturff, a Republican, and Jeff Horwitt, a Democrat.

In another sign of potential softness in the Democrats’ support, 58% of those who back Mr. Biden say their vote is more in opposition to Mr. Trump than in favor of their chosen candidate. By contrast, those planning to cast a ballot for Mr. Trump are more enthusiastic about their choice, with almost three-quarters saying their vote is more for him than against Mr. Biden.

“In one respect, Biden’s vote looks like Trump’s did in 2016: more a vote against their opponent than support for them,” Mr. Horwitt said.