Sen. Jeff Flake (R-AZ) is threatening to grind arguably the best-functioning part of President Donald Trump’s agenda to a halt.

The Arizona senator, a frequent Trump critic, indicated on Sunday that he would soon move to block consideration of the president’s judicial nominees until and unless Republican leaders allow votes on measures aimed at pushing back against Trump’s authority.

“I do think that unless we can actually exercise something other than just approving the president's executive calendar, his nominees, judges, that we have no reason to be there,” Flake, who is not seeking re-election, said on ABC’s This Week.

“So I think myself and a number of senators, at least a few of us, will stand up and say let's not move any more judges until we get a vote, for example, on tariffs,” he added.

Flake, along with several of his Republican colleagues, have been critical of the Trump administration’s trade policies, in particular the president’s use of a national-security provision to impose tariffs on U.S. allies.

Flake has co-sponsored a bill, authored by Sen. Bob Corker (R-TN), that would give Congress the authority to approve or strike down tariffs imposed under Section 232 of the Trade Expansion Act of 1962, which allows the administration to levy tariffs on imports that are deemed a threat to national security.

The senator’s ultimatum represents the last straw for a lawmaker who, along with Corker, has become increasingly isolated within his own party on Capitol Hill. Republican leaders have declined to bring the tariffs legislation up for a vote, with Senate Majority Whip John Cornyn (R-TX) arguing that it would not be prudent for Congress to pick a fight with the president so close to the midterm elections. Flake and his allies have soundly rejected that argument.

“We ought to more jealously guard our institutional prerogative,” Flake said. “I think in this crisis we're in, I think the judiciary has stood up well. The press has stood up well in terms of institutions. The balance. But the Congress has been lacking.”

Flake sits on the powerful Senate Judiciary Committee, where Republicans have a razor-thin 11 to 10 majority. If Flake joins with Democrats in voting to block Trump’s judicial nominees from advancing to the Senate floor, it would be a disaster for GOP leaders who have touted the record pace of judicial confirmations under Trump.