COLORADO SPRINGS — Big crowds still mob Donald J. Trump when he comes to town, with fans waiting in long lines to attend his rallies, where they eagerly jeer his Democratic rival and holler happily at his message.

But beneath the cheering, a new emotion is taking hold among some Trump supporters as they grapple with reports predicting that he will lose the election: a dark fear about what will happen if their candidate is denied the White House. Some worry that they will be forgotten, along with their concerns and frustrations. Others believe the nation may be headed for violent conflict.

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.”