Young people are a demographic in which Mr. Biden badly underperforms compared with most recent Democratic presidential nominees.

In Florida, voters under 35 are actually more likely to see Mr. Biden unfavorably than to hold a positive view of him. But 27 percent say they don’t know enough to feel one way or the other, so he has room to improve.

He leads Mr. Trump by just nine percentage points among Florida voters under 35, a relatively weak edge compared with Mrs. Clinton’s lead of 22 percentage points among this group in a Quinnipiac poll at this point in 2016.

Among millennials in Michigan, Mr. Biden squeaks by with just a 50 percent to 45 percent favorability rating among millennials, according to the Fox News poll. He leads Mr. Trump by just eight percentage points in a head-to-head matchup.