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(CNN) —  

Retiring Arizona Republican Sen. Jeff Flake said on Sunday that he has threatened to vote against federal judicial nominees because “it’s worth using a little leverage” to spur a vote on a bill to protect special counsel Robert Mueller’s Russia investigation.

In an interview with CNN’s Jake Tapper on “State of the Union,” Flake stressed that President Donald Trump’s newly appointed acting attorney general, Matt Whitaker, who now oversees the Mueller probe, “has not been confirmed by the Senate and … has expressed hostility to the Mueller investigation.”

“How in the world my colleagues don’t see this as a priority now, I just don’t understand,” Flake said of the legislation, which would protect the special counsel from being fired. “It does need to come to the Senate floor, and I think its worth using a little leverage here.”

“We need to do judges – we’ve done that on the floor … of the Senate,” Flake said. But he added that the bipartisan bill to protect Mueller was approved by the Senate Judiciary Committee in April – a rare bipartisan step that sends a warning signal to Trump not to remove Robert Mueller.

The legislation, which would give Mueller and other special counsels the ability to challenge their firings in court, has little chance of becoming law: Senate Majority Leader Mitch McConnell has vowed not to put it on the floor, House Republicans have shown no interest in the measure and Trump would be unlikely to sign it.

When asked if he would vote against a spending bill if a Mueller protection bill was not attached, Flake said that he would like to see the bill included in must-pass legislation, but he thought passing the separate bill first would increase its chances.

“I hope that [Democrats] continue to push for that, but the first step has to be having this bill [pass],” he said. “If we can do that first, there’s a far greater likelihood that it will be attached as part of the spending bill.”

CNN’s Jeremy Herb contributed to this report.