During the preparation for his second 2012 debate with Mitt Romney, I had to walk President Barack Obama through the conspiracy theories about the Benghazi attacks so he’d be ready for them: that he issued a “stand-down” order, for instance, to deny military assistance, or that we’d watched the attacks unfold in the White House via a drone feed. He didn’t believe me at first — unlike the current occupant of the Oval Office, he rarely watched cable news. “I’m serious,” I said. “It’s all over Fox.” Obama shook his head: “That’s some real tin-hat stuff.”

In the debate, Mr. Obama noted that he stood in the Rose Garden on Sept. 12, the day after the attack, and condemned it as an “act of terror.” Mr. Romney pounced, insisting repeatedly that the president had not done so. Mr. Obama, who had practiced his answer to this charge, replied, “get the transcript,” and the moderator, Candy Crowley, did just that, providing a rare real-time fact-check.

I watched the scene unfold backstage. Mr. Romney, a smart and serious man, reacted with disbelief. He was certain Mr. Obama had not called it an act of terror, even though the president had done so on national television from the Rose Garden. I shuddered a bit. I had grown used to Republicans using conspiracy theories to rile up their base. Watching Mr. Romney’s genuine shock, I confronted an even more disturbing reality: What if they actually believed these theories?

Mr. Obama may have gotten the best of that exchange. But having failed in 2012, the Republicans kept breathing oxygen into a meandering series of Benghazi conspiracy theories for four years — a trail that led all the way to Hillary Clinton’s private email server and, in some warped way, helped propel Donald Trump into the White House.