Republican Donald Trump’s performance in the first presidential debate on Monday night was likely to bolster his supporters, but risked turning others off, interviews with undecided voters and experts in both parties said.

“I feel that the way he talks to other people, the way that he addresses other people, can be extremely rude and extremely disrespectful, and I don’t think that’s the temperament we should be looking for in a president,” said Garrett Thacker, 30 years old, of Galloway, Ohio, who has voted for presidential...