Conspiracies surrounding Mr. Sanders’s political fortunes have been a particular fixation for Mr. Trump, dating back four years. During the 2016 campaign, Mr. Trump circulated the false and unsourced claim that an “analysis” — he did not say who wrote it or where it was published — concluded that Mr. Sanders would have won the Democratic nomination if not for superdelegates, the party leaders and officials who were not bound to vote for the winner of their states’ primaries or caucuses.

At the time, Mr. Trump and his advisers realized the potential political benefit in lobbing these kinds of accusations. Their campaign, which relied heavily on depressing Democratic turnout as a way to win battleground states like Florida and Michigan, stood to gain by fanning the flames of the rivalry between Mr. Sanders and Hillary Clinton and dredging up the bitterness that many Sanders supporters felt over their loss.

Even after winning the election, Mr. Trump continued to claim that Mrs. Clinton had somehow robbed Mr. Sanders of victory.