Trump's apparent attempt to determine whether Andrew McCabe would be loyal to him is the latest episode in the "relentless, all-out assault" on the FBI.

The evidence has been mounting for a year that Donald Trump may have obstructed justice in his attempts to shut down the Russia investigation.

But yet another shoe dropped Tuesday, when the Washington Post reported that the same week Trump fired FBI Director James Comey, he also questioned then-acting Director Andrew McCabe about how he voted in 2016. That seems to have been an attempt to determine whether McCabe might be more favorable to Trump or, like Comey, perceived as a threat.

Trump had unsuccessfully sought Comey’s pledge of loyalty —— a fact that McCabe, now deputy director of the FBI, corroborated during testimony last month before the House Intelligence Committee. Asking whether McCabe had voted for Trump or Clinton — a line of questioning that looks like an effort to determine McCabe’s potential loyalty — strikes an all too familiar and equally disturbing note.

“It is explosive, because what it shows is even more powerful evidence of obstruction of justice and interference with the FBI, and an attack on the FBI itself,” Connecticut Democrat Richard Blumenthal, a member of the Senate Judiciary Committee, told MSNBC’s Chris Hayes Tuesday evening.

He added that McCabe is “a longstanding career professional in the FBI,” and to ask him “whether he had political involvement in the sense of who he voted for … adds context also to the continuing work of the Judiciary Committee, which should be looking into whether or not this kind of interview involving the president of the United States and a career professional at the FBI represents an attack on this institution.”

The FBI, Blumenthal said, is “one of the premier law enforcement institutions in the world, and it is now under attack by Republicans — a relentless, all-out assault on law enforcement institutions.”

McCabe has become one of the top targets of attacks, smear campaigns, and conspiracy theories by Trump and his Republican enablers in Congress.

Trump’s interrogation of McCabe, immediately after firing Comey, only adds to the speculation that Trump has been trying to purge the FBI of those he deems insufficiently loyal and to obstruct investigations into him, his campaign, and his family.

And it’s a conversation that special counsel Robert Mueller is also investigating, according to the Post — further indicating that it could be a key piece of evidence in a potential obstruction case against Trump.