IN THE 2016 presidential race, then-candidate Donald Trump often deflected questions about his shortcomings by hyping those of his opponent, Hillary Clinton. The point was to make his behavior seem no more abhorrent than his rival’s.

Mr. Trump is trying the same tactic against presumptive Democratic nominee Joe Biden, latching onto a new diversion: “Obamagate.” This one is totally untethered from reality, despite the credulous or cynical efforts of senior Republicans and conservative media to advance it.

Mr. Trump’s precise allegation is hazy. But the apparent accusation is that, following Mr. Trump’s 2016 victory, Obama officials plotted to ensnare the incoming Trump administration in a Russia scandal.

Shortly before Mr. Trump’s inauguration, top Obama officials were told about phone conversations between Michael Flynn, a senior Trump national security aide, and Russian Ambassador Sergey Kislyak. In one, Mr. Flynn discussed sanctions the Obama administration had placed on Russia in response to Kremlin election interference. Mr. Flynn counseled Mr. Kislyak to refrain from retaliating, telling him that “we’ll review everything,” according to Mr. Flynn’s own account.

Obama aides asked that Mr. Flynn’s name be “unmasked” — that is, revealed to those seeing the intelligence reports, as it was redacted in the initial versions. That is not unusual; Trump administration officials have unmasked names thousands of times. Meanwhile, FBI officials kept open their investigation into Mr. Flynn, and he subsequently lied to them about his conversations with Mr. Kislyak — a federal crime to which he pleaded guilty.

Mr. Trump and his allies connect these dots in absurd ways, in an attempt to excuse Mr. Flynn and slime the Obama administration, including Mr. Biden, who was one of the officials who asked for Mr. Flynn’s unmasking. “Americans have a right to know the depth of Biden’s involvement in the setup of Gen. Flynn to further the Russia collusion hoax,” Brad Parscale, Mr. Trump’s campaign manager, said Wednesday. Attorney General William P. Barr did his part, moving on May 7 to drop charges against Mr. Flynn, despite his guilty plea.

Yet unmasking is a common and legal practice for those who have the proper clearance, as Mr. Biden and other Obama officials did. There is no evidence that Mr. Obama, let alone Mr. Biden, ordered any “setup” of Mr. Flynn. Indeed, one of the documents Obamagate conspiracists point to indicates that Mr. Obama wanted all matters relating to the Russia probe handled “by the book.”

Obamagate theorizers also have no good explanation for why Mr. Flynn lied to the FBI and to other Trump officials about his discussions with Mr. Kislyak. If there had been an Obama plot to set up Mr. Flynn, it could have been thwarted if Mr. Flynn had not broken the law. His crime was serious: He endangered national security, opening himself to Kremlin blackmail when he lied.

Mr. Trump would like to rewrite history, making it seem as though the Russia investigation lacked firm grounding and that a sinister Obama administration plot explains the misdeeds that his own people committed. The more Americans hear about Obamagate, the more they should wonder why Mr. Trump is not running on his record but on a dishonest effort to drag his opponent down to his level.

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