Arizona Republican Jeff Flake halted an appeals court nominee last week and he’s not publicly saying why, a behind-the-scenes move that takes aim at one of Senate Majority Leader Mitch McConnell’s top priorities this year.

The Senate Judiciary Committee scuttled a planned vote June 14 on the nomination of Britt Grant, a pick from Georgia for the U.S. Court of Appeals for the 11th Circuit. Flake’s hold throws a wrench in what has been the Senate GOP’s smoothly operating judicial confirmation machine.

“Oh, it’s just something I’m working out,” Flake said Tuesday, without offering more details. The typically media-friendly senator did say his reason didn’t have anything to do with Grant herself.

The move has not gone over well with Sen. Johnny Isakson, who said he confronted Flake but doesn’t know the reason for the hold. “It’s an indiscriminate, irresponsible use of a privilege of the Senate,” the Georgia Republican said. 

The issue could come up again as soon as Thursday, since Grant is listed again on the Judiciary Committee’s business agenda. 

Flake has been among the most outspoken congressional Republican critics of Trump, whose recent policies on immigration and trade have drawn judgment from his own party.

Separately, just before Flake’s move, some Senate Republicans aired concerns to McConnell about not holding floor votes on amendments to the fiscal 2019 defense authorization bill. When asked if the hold was related to that amendment issue, Flake repeated himself, saying only, “It’s something we’re working out.”

Isakson said he believes in using the hold only when it’s absolutely essential. “I went and confronted him and said I really needed him to let her go. She’s a terrific judge,” he said. “[Flake] has no reason for her not to go forward.”

McConnell has said moving Trump’s judicial nominees is a priority this year. The number and speed of Trump’s appointments to the nation’s circuit courts, which have the last say in all but the dozens of cases the Supreme Court decides each year, has been a point of accomplishment for the Kentucky Republican.

So far, the Senate has confirmed 21 of Trump’s appointees to appeals courts — or nearly 1 in 8 circuit judges in the country. Majority Whip John Cornyn of Texas said this month that Senate Republicans plan to move all of Trump’s appeals court nominees through the confirmation process and to the federal bench by the end of the year.

A McConnell spokesman did not respond to a request for comment. 

Flake’s move came late enough last week that Judiciary Chairman Charles E. Grassley made opening remarks about the panel voting on Grant, “a well-respected judge and public servant” who “has a clear record of being fair and impartial.”

But Grassley never called for the vote on Grant. Later that day, he wasn’t aware of why his staff told him not to do so.

“I don’t have a reason,” Grassley said after the June 14 business meeting. “They just said that we didn’t have the votes and so we shouldn’t bring it up. That’s the only reason I know, and there may be another reason.”

That vote total suggests all the Democrats are expected to vote against Grant, since Republicans hold an 11-10 advantage on the committee.