However, despite trying to change the claim in his interview with Bartiromo, it’s clear that Trump was alleging improper surveillance, not unmasking of legal surveillance, based on the way he has discussed it since the tweet. He made clear during a press conference on March 17, for example, that he was basing his accusation of surveillance on comments made by Andrew Napolitano, a Fox News legal analyst. Other reports have connected Trump’s tweets to a Breitbart article based on a Mark Levin radio segment. None of these focused on unmasking.

The unmasking defense flows from the mysterious intelligence reports that House Intelligence Committee Chairman Devin Nunes announced in March. Nunes, apparently working from documents given to him by two White House staffers, claimed that Trump transition-team members had been caught up in surveillance of foreign nationals. This did not justify Trump’s claim, because the alleged surveillance was legal, Nunes said, and had occurred after the election rather than before it.

The allegation of wrongdoing was more remote. When Americans are named or participate in surveilled conversations, their names are redacted. Top officials can request that their names be unredacted, or “unmasked,” if they believe it’s necessary to understand the report. Nunes said he was worried that Trump team members had been improperly unmasked. Bloomberg View’s Eli Lake later reported that Trump officials believed Susan Rice, Obama’s national security adviser, had requested that names be unmasked. Based on that revelation, Trump told the New York Times, again without evidence, that Rice had committed a crime.

Meanwhile, Nunes was forced to step down from the House investigation into Russia interference in the election after ethics complaints about his revelation of classified material and scrutiny of his relationship with the White House.

In trying to point to Rice, Trump is once again altering his claim in an effort to make it true. Yet not only is he shifting his claim after the fact, there’s no proof that what Rice did was improper. Rice was would have had the authority to ask for names to be unmasked (a request that the agencies would have had to approve) and, as Bartiromo pointed out, Rice says she would not have done so for political reasons.

“Does anybody really believe that?” Trump said. “Nobody believes that, even the people that try to protect her in the news media. It's such a big story and I'm sure it will continue forward.”

In fact, Trump appears to be wrong about that as well. Since Nunes’s recusal, Democrats and Republicans on the House Intelligence Committee have been able to view the documents he saw, and which he had not shared. CNN reports that they’re unimpressed with the results, which show only routine requests from Rice and anyone else in the Obama White House:

After a review of the same intelligence reports brought to light by House Intelligence Chairman Devin Nunes, both Republican and Democratic lawmakers and aides have so far found no evidence that Obama administration officials did anything unusual or illegal, multiple sources in both parties tell CNN. Their private assessment contradicts President Donald Trump's allegations that former Obama national security adviser Susan Rice broke the law by requesting the "unmasking" of US individuals' identities. Trump had claimed the matter was a "massive story."

Even when changing his story, Trump can’t land on a plausible one. The challenge to the president remains the same as it was on March 4: If he has any evidence for his explosive claims, he ought to produce it; and if he doesn’t, he ought to retract them.