In Democratic circles, the talk about the presidential election has increasingly turned to fears of a violent uprising by white supremacists and neo-Nazis if Donald Trump loses.

The nervous talk holds that Mr. Trump’s alleged racist followers are being primed to take to the streets to challenge election results should Democratic nominee Hillary Clinton prevail Nov. 8. They say Mr. Trump’s followers reveal their revolutionary fervor at rallies by chanting “lock her up” about the former secretary of state.

“Should we take the concern seriously? I think so,” said Ben Manski, a liberal activist leading the group No More Stolen Elections, which was formed in reaction to Republican George W. Bush’s win in the contested 2000 election.

“I don’t take neo-Nazis lightly. I have encountered them, and I am a Jew,” he said. “They don’t operate by the same set of rules that the rest of us do.”