Sen. Jeff Flake is serious about making Republicans vote on a bill to protect Robert Mueller.

Or is he?

Two weeks ago, Flake said that he would block all judicial nominees on the floor and in committee unless a bill protecting special counsel Robert Mueller received a Senate floor vote. And he proved that he was keen to follow through in a series of moves this week.

On Wednesday, Flake voted against advancing controversial nominee Thomas Farr, a North Carolina lawyer who is up for a district court spot, during a Senate floor vote. And since then, Judiciary Chair Chuck Grassley canceled a committee meeting that would have included votes on more than 20 judicial nominees, apparently due to Flake’s key vote in committee.

Senate Majority Leader Mitch McConnell has been clear that, aside from any spending bills, Republicans’ priority for the end of the year is to keep pushing judges through — and if Flake holds on this issue, it could throw a wrench into that plan.

But it’s not clear just how aggressive Flake is willing to be. On Thursday, he withdrew his opposition to a judicial nominee for the Eighth Circuit, a move he said was to balance out the absence of Sen. James Inhofe (R-OK), who was called away for family reasons, according to the Washington Post. Senators sometimes do this, as Sen. Lisa Murkowski famously did during Brett Kavanaugh’s confirmation when she withdrew her vote and was marked as “present.”

Flake’s move spurred questions about his backbone.

In the waning days of Flake’s presence in the Senate, the Arizona lawmaker, who’s frequently clashed with Trump, now has dwindling opportunities to make his mark. Longtime observers of his tortured reluctance to back Republicans going in a direction he does not like will recall that such stands often end with Flake simply, well, flaking.

It remains to be seen if he’ll live up to that standard one last time.

What does Flake want this time?

Flake has said he won’t back any judicial nominees until he sees a floor vote on legislation that protects special counsel Robert Mueller and his investigation into Russian election interference.

It’s an ultimatum that hits at Republicans’ narrow margins in the Senate (which will be wider when the new Congress starts in January).

As it stands now, Republicans need Flake’s vote to pass any nominations out of the 11-10 Judiciary panel for floor consideration. Senate Majority Leader Mitch McConnell does have the ability to simply move these nominations himself, but so far he has not seemed interested in making this practice a habit.

Similarly in the Senate, Republicans’ slim 51-49 majority means that Flake’s defection could be enough to help sink a nomination, or at the very least trigger a tie-breaker. In the case of Farr, Democrats have mounted an aggressive push to block his nomination. Given Flake’s current stance, if they can convince just one more Republican to switch over, they could successfully take it down.

Flake, along with Sens. Chris Coons (D-DE) and Cory Booker (D-NJ), attempted for the second time to schedule a floor vote on the bill protecting Mueller on Wednesday, but it was blocked by Sen. Mike Lee, who argued that the legislation was unconstitutional (a longstanding conservative argument, explained by Vox’s Tara Golshan here). The bill would ensure that Mueller could only be fired by a Senate-confirmed DOJ official and enable the special counsel to review any firing.

Many lawmakers see this legislation as increasingly more crucial in the wake of Attorney General Jeff Sessions’s departure and Trump’s ongoing critiques of the Mueller probe.

McConnell, however, has said he doesn’t see any threat of Trump firing Mueller, and he indicated Tuesday that he would “probably block” the bill from coming to the floor if Flake pushed it a second time. On the other hand, Majority Whip John Cornyn has previously signaled that there could be an openness to considering the legislation to keep things moving on judicial nominees, especially as the Senate seeks to wrap business up toward the end of the year.

If existing nominees don’t get confirmed before this congressional term concludes, they will have to be renominated by the White House in the new year. McConnell has emphasized that getting as many nominees through as possible is one of his key priorities this term and the next.

This isn’t the first occasion that Flake has threatened to withhold his support for judicial nominees. He did something similar before to prompt consideration of a measure on tariffs, though he ultimately relented after the Senate voted on a purely symbolic effort.

This time, though, he says he’ll only change course once the Senate votes on a binding bill. Now, it’s up to McConnell.