WASHINGTON – The Senate again blocked an expedited vote on legislation that would protect special counsel Robert Mueller from being fired.

Sens. Jeff Flake, R-Ariz.; Chris Coons, D-Del., and Cory Booker, D-N.J., urged brisk debate on the legislation in an effort to prevent President Donald Trump from firing Mueller Wednesday.

Republican Sen. Mike Lee of Utah objected to the request, blocking immediate action. Lee said the bill was unconstitutional.

Lee said if such a bill were passed it would “fundamentally (undermine) the principle of separation of powers" and create a "de facto fourth branch of government."

This was the second attempt to get a vote on the legislation. The bill had passed on a bipartisan basis out of the Senate Judiciary Committee, but has been kept off the floor by Senate Majority Leader Mitch McConnell, R-Ky., who said there is no need to bring it up.

Before calling for unanimous consent, Flake said he worried that his colleagues who said there was no need for legislation because "there hasn't been any indication that Mr. Mueller be removed from office" were making a mistake.

"With the president tweeting on a regular basis, a daily basis, that the special counsel is conflicted, that he is leading the so-called 12 angry Democrats and demeaning and ridiculing him in every way, to be so sanguine about the chances of him getting fired is folly for us, I believe," Flake said.

White House spokeswoman Sarah Sanders said Tuesday that Trump only wants the investigation to end and has no plans to somehow remove Mueller from his job.

"He could have taken action at any point and he hasn't,” Sanders said. “So we'll let that speak for itself.”

Sen. John Cornyn of Texas, the No. 2 Republican in the chamber, opened the possibility Tuesday of a vote on the Special Counsel Independence and Integrity Act, to ease votes on judicial nominees. Flake had vowed to vote against judicial confirmations unless there was a vote to protect Mueller.

"There is a possibility we will have a vote on the Mueller, so-called Mueller protection bill," Cornyn said.

But McConnell prevented an expedited vote earlier this month and said he would "probably" object if other senators tried to force a vote. Lee objected instead.

“This is a solution in search of a problem. The president is not going to fire Robert Mueller," McConnell told reporters Tuesday. "We have a lot of things to do to finish up this year without taking votes on things that are completely irrelevant to outcomes.”

But the bill may not be completely dead. Later Wednesday, Sen. John Thune, the current Republican caucus chair and incoming Whip, told reporters that leadership was still "trying to assess where our members are."

Flake, one of the few Republican critics of Trump, is retiring at the end of the year and has been working with Democrats to force a vote on the bill, even if it is unclear if it will pass.

The bill's supporters said protecting Mueller took on new urgency after the president fired Attorney General Jeff Sessions after Election Day and appointed Sessions' chief of staff, Matthew Whitaker, as acting attorney general. Sessions had recused himself from the Russia investigation and his deputy was handling it in his place. But with Sessions gone, Whitaker is now overseeing the probe.

The appointment worries some of Mueller's defenders, who point out that Whitaker had sharply criticized the investigation and suggested it could be defunded before he joined the Justice Department a year ago.

Contributing: Deirdre Shesgreen