McCabe will now take the seat that was to be filled by Comey Thursday and will likely be the target of some probing by senators angry over how Comey was fired by Trump. The questions about connections between the Trump campaign and Russian operatives are burning hotter than ever.
Here are the five things to watch at Thursday's hearing.
Just one day after Comey was fired, Senate judiciary committee Chairman Chuck Grassley was already lacing into his (potentially) temporary replacement, McCabe.
McCabe, Grassley said, is too much of a partisan in support of Democrats and will not be credible.
"He's got political problems because of (Virginia Gov. Terry) McAuliffe helping his wife and I don't think he's the person that should be taking over," Grassley told CNN's Manu Raju.
McCabe's wife ran for Virginia state senator as a Democrat in 2015, and was defeated by a Virginia Republican, something that may not raise any eyebrows in a typical setting -- but in this highly charged environment, almost everything is being questioned by lawmakers.
2. Will McCabe's oversight of the FBI end up being a moot point?
Almost as soon as he became acting director, McCabe was already facing a possibly tenuous run atop the the agency, as Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein interviewed four to eight candidates for the job of FBI director Wednesday.
The pair, a Justice Department official said
, could announce their nominee as soon as Thursday.
It could be a very short tenure for McCabe -- or it could be the start of a long relationship, if Sessions and Rosenstein decide McCabe is their man.
Senators will have a chance to test McCabe at Thursday's hearing. The White House, meanwhile, will likely be listening in on reactions to McCabe.
Whomever Trump picks will have to win the support of at least 50 Republican senators (with Vice President Mike Pence acting as a tie-breaking vote.) It's almost guaranteed Democrats will vote en masse against any Trump nominee, following their outrage over his handling of the Comey firing. Losing Grassley on a nominee would make the White House's job infinitely harder.
3. Is the White House looking for a strong presence in public?
Comey was, undeniably, a forceful director in public settings -- from his surprising announcement at the end of the Hillary Clinton email investigation through to his last public hearing as FBI director -- where he publicly announced his rationale for breaking from the Justice Department in making the decision to publicly speak about the Clinton matter, a move that ultimately contributed to his firing.
Watch McCabe to see how far he veers from the White House script, if at all.
4. Will Senate investigators be confident that McCabe or any Trump nominee will help them?
Until Tuesday, Comey was actively working with Senate Russia investigators, even meeting with them Monday to tell them that he had requested additional resources for his own probe.
And, unlike House investigators, members of the Senate intelligence committee have said they have never had any problems getting access to intelligence needed for their investigation.
That could easily change under whomever Trump nominates to replace Comey.
5. Comey's been invited back to the Senate, so how about Sessions and Rosenstein?
Senate intelligence Chairman Richard Burr and Sen. Mark Warner, the Russia Senate investigation's top Democrat, invited Comey to testify before them in a closed hearing Tuesday, now as a private citizen.
That would give Senate investigators Comey's side of the story. But what about the pair that recommended he be fired: Sessions and Rosenstein?
Democratic lawmakers have said they would like answers -- it remains to be seen if they will be pulled before Congress as well.