DELAWARE, Ohio—Donald Trump on Thursday sought to explain his debate-stage suggestion the night before that he might not accept a loss in November’s presidential vote as legitimate, saying at a rally here that he was simply affirming the ability of candidates to question an election outcome.

“I would also reserve my right to contest or file a legal challenge in the case of a questionable result,” Mr. Trump said. “I will follow and abide by all the rules and traditions of all of the many candidates who came before me, always.”

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