First, let's recap the weekend highlights:
On Saturday, the day after embattled Attorney General Jeff Sessions fired the retiring Deputy FBI Director Andrew McCabe, Trump for the first time tweeted out the name of his tormentor, saying, "The Mueller probe should never have been started in that there was no collusion and there was no crime. It was based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC, and improperly used in FISA COURT for surveillance of my campaign. WITCH HUNT!" (For fact-check of those claims, consult The Washington Post's Glenn Kessler.)
That same day Trump's personal lawyer, John Dowd, told The Daily Beast, "I pray that Acting Attorney General Rosenstein will follow the brilliant and courageous example of the FBI Office of Professional Responsibility and Attorney General Jeff Sessions and bring an end to alleged Russia Collusion investigation manufactured by McCabe's boss James Comey based upon a fraudulent and corrupt Dossier." (White House lawyer Ty Cobb has since insisted that the president "is not considering or discussing the firing of the Special Counsel, Robert Mueller.")
Meanwhile, Trump is out here tweeting that Mueller's team has "13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans" (WashPost context-check), and that it's all a "total WITCH HUNT with massive conflicts of interest!" and so forth.
The Republicans who have raised objections to the prospect of Trump trying to fire Mueller broadly fall into three categories: Hawks, Squishes, and Retirees (some of them libertarian-leaning). These blocs map pretty well onto the conservative opposition to Trumpism thus far, and therefore could be more influential than they are today come December 2018, should the GOP lose the House and Senate.
Leading off for the don't-go-there Hawks is Sen. Lindsey Graham (S.C.), who said on CNN's State of the Union Sunday that "If he tried to do that, that would be the beginning of the end of his presidency." More: "I think it's very important [Mueller] be allowed to do his job without interference, and there are many Republicans who share my view." These sentiments were shared, unsurprisingly, by Sen. John McCain (Ariz.): "Special Counsel Mueller has served our country with honesty and integrity. It's critical he be allowed to complete a thorough investigation into Russia's interference in the 2016 election—unimpeded."
In the finger-in-the-wind Squish category we can find House Speaker Paul Ryan (Wisc.), whose spokeswoman statemented that, "As the speaker has always said, Mr. Mueller and his team should be able to do their job." I guess it depends on the meaning of the word "should."
And among the Retirees—elected officials who have announced they are leaving at the end of their current terms—there is the increasingly off-the-reservation House Intelligence Committee member Trey Gowdy (S.C.), who advised the president on Fox News Sunday, "When you are innocent…act like it," adding: "If you've done nothing wrong, you should want the investigation to be as fulsome and thorough as possible."
The lead Retiree, as usual, is Sen. Jeff Flake (Ariz.), who on State of the Union said that firing Mueller would be "a massive red line that can't be crossed." The senator further insisted: "Talking to my colleagues all along, it was, 'Once he goes after Mueller, then we'll take action.'" So we'll see about that.
As for the McCabe firing, Flake said, "I think it was a horrible day for democracy….To have firings like this happening at the top, from the president and the attorney general, does not speak well for what's going on. I don't know what the designs are on for Mueller, but it seems to be building toward that, and I just hope it doesn't go there, because it can't. We can't, in Congress, accept that."
If anti-Trump sentiment in the broader electorate leads to a GOP wipeout this November—and the still-preliminary polling right now is pointing in that direction—then there will be yet another feverish battle to define the modern GOP. Already, Flake is making noises about a potential presidential run (on which more in this space later); if Democrats control Capitol Hill, he certainly would not be alone.