Whatever bipartisan momentum exists for a bill to protect special counsel Robert S. Mueller III from getting fired by President Trump just got squashed — by the one senator proponents of the legislation needed on their side, Senate Majority Leader Mitch McConnell (R-Ky.).

“This is a piece of legislation that's not necessary, in my judgment,” McConnell said in an interview Tuesday afternoon with Fox News's Neil Cavuto. "… I'm the one who decides what we take to the floor. That's my responsibility as the majority leader. And we will not be having this on the floor of the Senate.”

That was the most forceful refusal we've heard from McConnell about a bill to protect Mueller, which is expected to get a committee vote as soon as this week after overcoming months of hurdles in behind-the-scenes negotiations.

Given that Trump has made very clear he does want to fire Mueller, rejecting such a bill is a risky move on McConnell's part.

But it's a risk McConnell seems willing to take, for a variety of reasons.

McConnell, like most Republicans in Congress, has walked a fine public line on the president's on-again, off-again idea of firing Mueller or the person who appointed him, Deputy Attorney General Rod J. Rosenstein. McConnell doesn't think the president should fire Mueller, but he also doesn't think the president will fire Mueller.

GOP leaders in Congress have repeatedly cited vague, anonymous assurances from “the White House” that make them feel confident Mueller's job is safe. If we take them at their word, then these Republican lawmakers know something the rest of us don't about fail-safes in place to stop Trump from doing something that could launch a constitutional crisis. (Trump technically doesn't have the authority to fire Mueller, nor good reason to fire Rosenstein other than to get the Russia investigation off his back.)

McConnell also made the point Tuesday that trying to pass this bill would be pointless: “Just as a practical matter, even if we passed it, why would he sign it?” McConnell told Cavuto.

Of course, Congress has been able to force Trump to reluctantly sign bills before, like a Russia sanctions bill that passed Congress by an overwhelming veto-proof majority.

The bottom line is that McConnell seems to think there's a greater risk to considering this Mueller bill than not considering it. The Russia investigation and its spinoff legal dramas are perhaps the biggest crisis Trump faces right now. My Washington Post colleagues report that Trump had trouble concentrating last week on Syria military strategies after he learned the FBI raided his personal lawyer's office and residences.

Under that context, if Congress antagonized Trump by passing a bill that basically says lawmakers don't trust him, it would risk open warfare with the president. That's something GOP leaders in Congress have tried very, very hard to avoid.

A breakdown in that relationship would be especially brutal for Republicans during an election year when polls show the Republican base loves Trump. They need the president to sign their bills into law so they can go campaign on those laws. And Republican lawmakers need Trump on their side politically to get their voters out to help them keep their congressional majorities in an increasingly difficult electoral environment for them.

McConnell appeared to get at that dynamic in his Cavuto interview by saying he is going to focus on passing legislation that Trump likes: “I'm going to concentrate here in the Senate and have my colleagues voting on things that are relevant to moving the ball toward the goal line on every single issue that we think we can pass and put on the president's desk between now and November.”

Protecting Mueller is not a priority for McConnell this November. It's a decision he has clearly already made, but one that carries significant risk if he's flat-out wrong on his premise.