His theory—which others view skeptically—holds that fear of social stigma prevents some voters from admitting they back him, including when talking to pollsters. Last week he called the polls “crooked” and said, “The whole election is being rigged.’’

“This is the only realistic sketch for a Trump victory, and I think there’s some truth to it,” said David Hill, a veteran Republican pollster. He said he saw evidence of Trump voters reluctant to admit their support while working on Jeb Bush’s GOP primary campaign. “But expecting this will be enough to make up 3 points or more?” Mr. Hill said. “That’s a bridge too far.”…

Pollster Whit Ayers, who has worked for Sen. Marco Rubio (R., Fla.), said any hidden support for Mr. Trump likely would be balanced by “Hillary Republicans” who don’t want to admit their preferences publicly.

In the latest Wall Street Journal/NBC News poll, released Sunday, 13% of those surveyed said they knew people who supported the Republican nominee but were hesitant to say so, because they worried what others might think. At the same time, 8% said they knew people who were voting for Mrs. Clinton but were afraid to disclose their preference.