Sen. Jeff Flake, the lame-duck Arizona senator who’s long clashed with President Donald Trump, is once again threatening to use his position to express concerns about executive power and the fate of the Russia investigation.

It’s not an empty threat. But it’s not yet clear to what extent he’ll follow through.

Flake said on Wednesday that he would oppose all judicial nominees coming through the Senate Judiciary Committee and the Senate floor until Majority Leader Mitch McConnell puts a bill protecting special counsel Robert Mueller up for a vote.

“I have informed the majority leader I will not vote to advance any of the 21 judicial nominees pending in the Judiciary Committee or vote to confirm the 32 judges awaiting confirmation on the Senate floor until ... [the bill] is brought to the full Senate for a vote,” Flake said in a speech at the Capitol.

In making this threat, he’s attempting to hit McConnell on something he cares about quite a bit: his goal to push through as many conservative judicial nominees as possible. It’s something the leader has emphasized repeatedly and a message he came back to on Wednesday while laying out his congressional priorities.

The legislation, a bipartisan bill from Sens. Lindsey Graham (R-SC), Chris Coons (D-DE), Thom Tillis (R-NC), and Cory Booker (D-NJ), would ensure that the special counsel can only be fired by a Senate-confirmed Justice Department official. It would also enable Mueller to request a formal review into any kind of potential removal and guarantee the preservation of staffing and materials tied to the special counsel’s investigation. The bill passed the Senate Judiciary Committee in April, but McConnell refused to let it on to the Senate floor when Flake and Coons tried to force a vote on Wednesday.

Without Flake’s vote, it’s possible that nominees would not be able to advance out of the Judiciary Committee given the narrow one-person Republican majority on the panel. If Flake withheld his vote on the floor, Republicans would also likely have to use Vice President Mike Pence as a tiebreaker as judicial nominees come up for consideration.

In both cases, however, there are ways for McConnell to just barrel forward: He can move nominees even if they haven’t been approved by committee, and he could likely keep the Republican conference united on most judicial picks.

Flake’s stonewalling could definitely make things a bit more complicated. What remains to be seen is what his actual execution will look like.

Flake has, after all, used this kind of threat in the past, only to ultimately soften his opposition. Most recently, he expressed concerns about sexual assault allegations against Supreme Court Justice Brett Kavanaugh and even said he wouldn’t vote on the nominee until an FBI investigation was conducted — only to wind up very anticlimactically supporting his confirmation after a rushed investigation.

He’s also levied the threat of voting against Trump’s appellate court nominees if action wasn’t taken related to tariffs, opposition he dropped after the Senate voted on a purely symbolic measure related to trade.

It’s unclear how far he’ll go this time around.