Jared Halbrook, 25, of Green Bay, Wis., said that if Mr. Trump lost to Hillary Clinton, which he worried would happen through a stolen election, it could lead to “another Revolutionary War.”

“People are going to march on the capitols,” said Mr. Halbrook, who works at a call center. “They’re going to do whatever needs to be done to get her out of office, because she does not belong there.”

“If push comes to shove,” he added, and Mrs. Clinton “has to go by any means necessary, it will be done.”

Interviews with more than 50 Trump supporters at campaign events in six states over the past week revealed a distinct change from the rollicking mood earlier this year, when Mr. Trump’s surprising primary successes and emergence as an unconventional Republican standard-bearer set off broad excitement. The crowds appeared on edge and quick to lash out.

And while some voters emphatically disputed polls suggesting that Mrs. Clinton would win, others offered an apocalyptic vision of what life would be like if she did.