Meanwhile, Mr. Trump’s approval rating has risen, most likely lifted by the strength of the economy and his acquittal in the Senate’s impeachment trial. His Gallup approval rating hit 49 percent this month — the highest number of his presidency. So while a majority of Americans still disapprove of him, he has the potential to win at least as much support as he did in 2016, when he lost the popular vote but pulled off a decisive win in the Electoral College.

The ABC/Washington Post poll showed that in head-to-head matchups with all six of the candidates onstage at the debate, Mr. Trump would earn between 45 and 47 percent of the vote nationwide. The Democrats’ shares varied from 49 to 52 percent. The differences between each Democrat were all within the poll’s margin of error.

This suggests that, despite Mr. Sanders’s claim that a “political revolution” would be needed to drive turnout, and in spite of his more moderate rivals’ argument that his left-wing proposals would alienate a huge swath of voters, any Democratic nominee would probably have a good chance of winning the popular vote.

But Republicans have won the Electoral College three times this century, even though they have won the popular vote only once. So what really matters is who can beat Mr. Trump in a handful of key swing states.

That’s why Quinnipiac University asked voters in Michigan, Pennsylvania and Wisconsin — three states that Mr. Trump narrowly flipped in 2016, giving him an Electoral College win — to choose between the president and each of the six leading Democratic candidates.

The data from those polls suggests that Mr. Trump is in a very strong position to win Wisconsin, a heavily white state that was once solidly blue but has trended conservative over the past 10 years. None of the six leading Democratic candidates finished ahead of Mr. Trump there.

Conversely, in Michigan and Pennsylvania, he didn’t have a lead against any of them. Mr. Biden beat Mr. Trump by eight percentage points in Pennsylvania — where he was born — and by four points in Michigan. Mr. Sanders was up by five points against the president in Michigan, and by four points in Pennsylvania. Mr. Bloomberg led Mr. Trump by six points in Pennsylvania, and by five points in Michigan.