The danger for Mr. Trump among Hispanic voters could be greater in Florida than anywhere else. Not only are there more Hispanic voters than in any of the other big battleground states, but the Hispanic vote has also been unusually Republican. Put it together, and 6 percent of Florida’s voters in 2012 were Hispanics who supported Mr. Romney. There’s a lot of room for the G.O.P. to fall if it loses a big chunk of those voters.

The more surprising part of Mr. Trump’s problem is the white vote. He’s winning the state’s white voters by 19 points — slightly below Mr. Romney’s 20-point edge in similar estimates from 2012, even though the state’s white vote is not especially well educated (Trump has generally been performing well nationally among whites without college degrees).

Why is Mr. Trump struggling with white Floridians? Mrs. Clinton appears to be running ahead of Mr. Obama among older and Southern white voters, reversing the one trend that has helped Republicans over the last decade.

This might have been a problem for the Republicans even if Mr. Trump and Mrs. Clinton were not the nominees. The G.O.P.’s gains among white voters in the state, particularly in the Florida Panhandle, appeared to slow in 2012 — allowing Mr. Obama to hold a state he carried by just three points in 2008.