Earlier this month, Representative Adam Kinzinger told his constituents that he is worried by the excessive number of conspiracy theories he has seen circulating lately on social-media sites.

“As leaders, we have a choice,” he told his constituents in a video message posted to Facebook. “There’s far too many who will simply reflect back that paranoia, to feed fuel to that fire, if it’ll help their reelection … We need to push back against these attempts to divide and destroy us.”

Days later, President Donald Trump disseminated a conspiracy theory for the ages. In the midst of an ongoing pandemic that has killed 100,000 Americans, he attacked a cable-television host, former Representative Joe Scarborough, with a thinly veiled murder allegation. “A lot of interest in this story about Psycho Joe Scarborough,” Trump wrote. “So a young marathon runner just happened to faint in his office, hit her head on his desk, & die? I would think there is a lot more to this story than that? An affair? What about the so-called investigator?”

In fact, there is no evidence of an affair or […]