WASHINGTON (Reuters) - A lone Republican lawmaker on Wednesday vowed to block judicial nominations in the U.S. Senate until Republican leader Mitch McConnell agrees to hold a vote on a measure to protect a probe into Russia’s role in the 2016 presidential election.

Senator Jeff Flake made the threat after McConnell blocked Flake’s measure from moving forward. The Special Counsel Independence and Integrity Act would make it harder for President Donald Trump to undermine the investigation into Russian meddling and any coordination between Moscow and the Trump campaign.

Moscow denies interfering and Trump has repeatedly dismissed Special Counsel Robert Mueller’s investigation as a political witch hunt.

“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,” Flake said in a floor speech after his effort to win consent for a vote failed.

Flake, who is retiring from the Senate, used the same strategy earlier this year to obtain a Senate vote on an amendment to rein in Trump’s power to impose tariffs on foreign goods.

His vote is especially crucial on the 21-member Senate Judiciary Committee, which Republicans control by only a single seat. But Flake could also make life difficult for McConnell on actions in the 100-seat Senate, which Republicans control by only a 51-49 seat margin through the end of the year.

McConnell has maintained for months that there is no danger to Mueller’s investigation.

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“I don’t think any legislation is necessary,” he told reporters after winning his bid to remain Senate Republican leader.

“We know how the president feels about the ... investigation, but he’s never said he wants to shut it down,” McConnell said.

However, on Aug. 1, Trump appealed to then-U.S. Attorney General Jeff Sessions to end the investigation, sending the following message on Twitter: “This is a terrible situation and Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now.”

McConnell came to the Senate floor himself to prevent Flake’s bill from moving forward.

Flake and Democratic Senator Chris Coons, also part of the effort to obtain a vote to protect Mueller, said they would continue to seek legislative action and could push to have a measure included in a government spending bill later this year.

A week ago, Trump set off alarm bells among Democrats and some Republicans by forcing the resignation of Sessions and replacing him with Matthew Whitaker, a Trump loyalist who has criticized the Mueller probe as too far-reaching.

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Critics of the Whitaker appointment say that, as acting attorney general, he could fire Mueller or undermine the Russian meddling investigation in some other way.

The bill backed by Flake and Coons would ensure that Mueller could be fired only for good cause and provide him with recourse to challenge any dismissal in federal court. It was approved by the Senate Judiciary Committee in a 14-7 vote in April.

It is supported by the panel’s chairman Chuck Grassley, as well as prominent Senator Lindsey Graham. Both are Republicans.

It is also backed by Democrats. “We want to make sure that the acting attorney general does not interfere with the Mueller investigation, and we believe our Republican colleagues can join us,” Senate Democratic leader Chuck Schumer told reporters.

Flake said there were enough votes to pass the bill. He said there would be repeated efforts to bring it to a vote despite McConnell.