President Trump has long used his fame and platform to amplify conspiracy theories and undermine his political enemies by muddying the waters when it comes to facts.

For years, he has helped to erode voter faith in institutions by invoking the idea of a sinister force — such as the “deep state” or a rigged electoral system — that is thwarting the will of the people in an attempt to undermine him.

But a macabre subgenre of Mr. Trump’s fondness for conspiracy theories has been to accuse, directly or indirectly, his political enemies of murder.