Many have laughed at Trump's concerns, but on Sunday, Deputy Attorney General Rod Rosenstein gave momentum to President Trump's belief that the FBI may have spied on his 2016 presidential campaign for political reasons. Rosenstein did so by ordering the Justice Department's inspector general to investigate whether there has been "impropriety or political motivation" in the FBI's Trump-related investigations.

Trump had called for just this kind of investigation earlier on Sunday with the following tweet:

I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes - and if any such demands or requests were made by people within the Obama Administration! — Donald J. Trump (@realDonaldTrump) May 20, 2018



But whatever happens next, Rosenstein's expansion of the inspector general investigation, which is already investigating possible mishandling of the Foreign Intelligence Surveillance Act application process, is something of a vindication for Trump.

After all, Rosenstein isn't just some guy who wants to tick boxes for Trump. He knows that by ordering an expansion of the inspector general's investigation, he is is staking his professional, non-partisan credibility on the belief that such an expansion is necessary. As a servant of the law in a period of immense political controversy, Rosentstein's first responsibility is to ignore political pressure, which Trump's tweet would represent if it was without objective merit.

Rosenstein is implicitly telling us that he believes the president has at least a probable cause for that concern. Inspector general investigations are serious matters that are not supposed to be ordered on a whim. Indeed, to order such an investigation without merit would put Rosenstein in the crosshairs of an inspector general investigation.

As an extension, the deputy attorney general's decision is bad news for Democrats. They have been arguing persistently that the president is crying wolf for the purpose of political distraction. And to make that case, Democrats and their media allies have pointed to Rosenstein's professional credibility as a legal servant beyond reproach.

Rosenstein's latest action thus puts Democrats in a catch-22.

They could continue supporting Rosenstein as the voice of objective reason, and thus effectively endorse the legitimacy of Trump's concerns, or to turn against Rosenstein, which would suggest their own motives might just be partisan.