In the third and final presidential debate, Republican Donald Trump’s refusal to say he would accept the results of the election was the last straw for at least some voters who had been struggling with their decision.

Bijan Sharifi, a 34-year-old high-school teacher, said he was horrified by Mr. Trump’s comment during the debate that “I will tell you at the time” whether he would abide by the election’s outcome. Mr. Trump has been warning for months about a “rigged election” and has stepped up those allegations recently as...