Senate Republicans on Wednesday blocked a second attempt to quickly pass legislation aimed at protecting special counsel Robert Mueller from being fired without cause by President Trump.

Sens. Jeff Flake, R-Ariz., Chris Coons, D-Del., and Cory Booker, D-N.J., pushed the Senate to accept their legislation by unanimous consent. They and other senators worry that Trump could be on the verge of firing Mueller while he's still in the process of investigating Trump's alleged ties to Russia.

It wasn't entirely clear what Republicans would do on Wednesday, as Senate Majority Whip John Cornyn, R-Texas, said on the Hugh Hewitt radio program Tuesday morning that McConnell might allow the resolution to come to the floor to appease Flake, who is withholding support for judicial nominations in a bid to get a vote on the Mueller bill.

But when Flake asked for unanimous consent to approve the bill, Sen. Mike Lee, R-Utah, blocked the move, just as Senate Majority Leader Mitch McConnell did on Nov. 14. Lee said the legislation is a breach of the separation of powers, and is thus unconstitutional.

“Prosecutorial authority of the United States belongs in the Department of Justice, the Department of Justice answers to the President of the United States. It’s principal officers consistent of people appointed by the president serving at the pleasure of the president after being confirmed by the U.S. Senate,” explained Lee. He argued that Flake's bill would essentially create a new branch of government that isn't answerable to the president.

“We cannot convert an office like this one, an office like the previously existing office of independence counsel without creating a de facto fourth branch of government fundamentally undermining the principal of separation of powers that is so core to our liberty,” said Lee.

Without unanimous consent, the Senate needs to go through regular order, and McConnell has made it clear he won't call up the bill that way. McConnell indicated late Tuesday that Republicans would block the effort by saying there's no sign Trump is about to move against Mueller.

"The president is not going to fire Bob Mueller, nor do I think he should, nor do I think he should not be allowed to finish," McConnell told reporters on 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."

The legislation to protect Mueller would codify current Justice Department rules that say a special counsel cannot be removed without good cause. Flake argued that the bill "serves one purpose: that is to protect the integrity of the special counsel's investigation, and to prevent the Executive Branch from inappropriately interfering in an independent investigation in the future."

About an hour after the vote, Flake made good on his vow to vote against all of Trump's judicial nominations, and forced Vice President Pence to break a 50-50 tie to advance one of Trump's picks.

Coons said on the Senate floor that Trump firing Mueller would spark a constitutional crisis that would threaten the presidency and the rule of law, and said the bill would stop that from happening. “We can work together to prevent a crisis," he said.

The GOP-led Senate Judiciary Committee approved a bill last spring that would make it harder for Trump to fire Mueller, in a bipartisan 14-7 vote.

Flake and others have pushed for it in the last few weeks, especially after the ouster of former Attorney General Jeff Sessions, which some fear could be a sign Trump will next target Mueller.

Acting Attorney General Matt Whitaker, a known ally of Trump, is now in charge of oversight of Mueller’s investigation.

Despite support from some GOP lawmakers, top Republicans like McConnell and Cornyn have said the bill is unconstitutional.

On Wednesday, Coons told MSNBC’s “Morning Joe” that McConnell is working to protect Trump by blocking the legislation.

“I think, frankly, at the end of the day, Leader McConnell has gotten reassurances from the president that he won't act against Mueller, but those assurances are undermined every single day when President Trump both tweets untrue criticisms of Robert Mueller and his investigation and does other things that are unexpected or unconventional or unjustified,” Coons said.

Coons said there are no plans in place now to tie the Mueller bill to must-pass legislation, such as a spending bill that needs to pass by next Friday to avoid shutting down swaths of the government.