JD Vance wanted to focus on the future, he said. His past, and his party’s, intervened.

For some 90 minutes, Mr. Vance, a proud Republican ambassador to the online right, had largely tailored his debate-night message to a mass audience, avoiding most detours into conservative fever swamps, as if determined to deliver a rolling rebuttal to Democrats’ longstanding suggestion that he was “weird” and out of step.

But when the debate turned, near its final frames, to the subject of the 2020 election, Mr. Vance faced a choice: He could validate, once more, Donald J. Trump’s relentless lies about his defeat four years ago. Or he could try something else in the spirit of moving forward.

It did not seem like a difficult decision for him.

“What President Trump has said is that there were problems,” Mr. Vance said when asked about his own past assertion that he would not have certified the 2020 election. “We should fight about those issues, debate those issues, peacefully in the public square. And that’s all I’ve said. And that’s all that Donald Trump has said.”

His debate opponent, Tim Walz, stared at him, unblinking, and then looked down at his lectern.

“Remember,” Mr. Vance said of Mr. Trump, “he said that on January the 6th, the protesters ought to protest peacefully. And on January the 20th, what happened? Joe Biden became the president. Donald Trump left the White House.”