Remember that McConnell isn't going to let this get a floor vote, and the House? They won't even schedule committee action on it. This bill is dead, dead, dead. So, why are Jeff Flake, Lindsey Graham and Thom Tillis bothering?
Some possibilities:
1) They think they are "sending a message," or some such crap, to Trump. If so, they are wrong. The message to Trump is the fact that the bill won't get a floor vote, demonstrating that he can fire Mueller. If the point were to send a message, the message would need to be, "fire Mueller, and here's what I will do afterwards." Notice that we aren't hearing that. This is why I don't actually think this is what is going on.
2) They are attempting to signal partisan independence to voters. You have probably heard and read a bunch of stuff about how everyone in the GOP is scared of primaries. Empirically, though, extremists face bigger penalties in general elections. The McCain trick was always to pick a fight on a few things with his own party, and that would allow him to get away with a lot. Notice who's in this group? McCain's little buddy, Lindsey Graham! Of course, Flake is retiring, so that doesn't really explain him.
3) Perhaps they are simply urging Trump to wait before firing Mueller. Look, Lindsey Graham will let himself get beaten into submission by Trump. He always does. If Trump fires Mueller, Graham won't stand up to him. Frankly, neither will Flake. Remember how Flake was going to vote against the tax bill because it raised the deficit? Yeah, how'd he vote on that? Oh, right. However, there is a reasonable chance that Mueller's investigation will get to a point at which Trump will need to fire him in order to protect himself, and the party will need to back Trump in order to protect themselves because, as I keep writing, Trump has an electoral bomb strapped to him, wired up with a dead-man trigger, and the entire party is chained to him. That is the point at which the party will back him on firing Mueller. They'll all claim he's a commie-ISIS agent, secretly orchestrating the conspiracy to falsify climate change data so that we accept fluoride in our water to sap and impurify all our precious bodily fluids, Mandrake! (Do I have to link to that one? Come on, people, it's a classic.) Then, they'll let him fire Mueller, but not one second earlier.
4) Ineffectual, whimpering, pathetic attempts by spineless people to convince themselves that they will fight when they know that they will, eventually, cave. Donald Trump doesn't have many skills. He does have one skill: bullying. He really is great at it. He is outstanding at it. Lindsey Graham always caves in the face of his bully, as does Jeff Flake. Tillis has never bothered trying to stand up to Trump, and neither have most congressional Republicans. They really are a weak and pathetic bunch. Nobody wants to feel like that much of a victim, though. So, Lindsey, Thom and Jeff are puffing up their chests and hoping that Trump doesn't fire Mueller, but that's about it.
At the end of the day, this bill is meaningless, as are the efforts of any Republican supporters because it has no chance of passage. The only real question anyone should ask any Republican who claims to support the bill or oppose Mueller's firing is the following: what will you do if and when Trump fires Mueller? Nobody should be allowed to use McConnell's dodge.
What's the penalty?
And here's the problem. Trump will make up some bullshit excuse for why he did it, and every Republican will go along with it. Any Republican who answers my question with any wiggle room will claim that Trump's reason is within that wiggle room.
And that's what's going on.