If you doubt Republicans are facing immense pressure these days, consider Sen. Martha McSally’s behavior. Asked by respected, mild-mannered CNN reporter Manu Raju if she would consider new evidence during the impeachment trial of Donald J. Trump, the vulnerable senator from Arizona snapped back: “You’re a liberal hack—I’m not talking to you. You’re a liberal hack.”

Regardless of whether new post-House impeachment revelations are introduced in the Senate trial, the drip-drip-drip has created a lose-lose proposition for Republicans who face tough electoral crosswinds in 2020. They can defend the indefensible, or they can risk invoking the wrath of their president. (Clearly, McSally has decided her best bet is to avoid the latter.)

But as evidence mounts, McSally also risks alienating Arizonans who elected a maverick named John McCain and aren’t looking to send a Trump toady to Washington.

As the Senate heads toward the formal impeachment trial of Donald J. Trump on Tuesday, text messages and documents provided by Lev Parnas, as well as a new Government Accountability Office (GAO) report showing that the White House broke the law by withholding funds from Ukraine, cast a pall over Republican efforts to pretend this is merely a witch hunt.

Of course, Republicans who do not face these crosswinds have different incentives. Trump can lose the popular vote and still win the Electoral College, by narrowly holding Pennsylvania, Michigan, and Wisconsin by the skin of his teeth. He does that, potentially, by holding his base. But that would be cold comfort for Republicans trying to hold Senate seats in places like Maine (Collins) and Arizona (McSally), and also states like Colorado and North Carolina.

With a different leader, these vulnerable senators might be afforded the opportunity to subtly distance themselves from this president. But Donald Trump demands complete loyalty and attempts to walk the line between appeasing the Trump-loyal GOP base and wooing suburban swing voters are being made impossible by fellow Republican senators like Rand Paul, who, in the words of Politico, is “vowing to squeeze vulnerable GOP incumbents” if they support procedural motions to allow for the calling of witnesses during Trump’s impeachment trial. “If you vote against Hunter Biden, you’re voting to lose your election, basically. Seriously. That’s what it is,” Paul said Wednesday. “If you don’t want to vote and you think you’re going to have to vote against Hunter Biden, you should just vote against witnesses, period.” With friends like these...

For now, at least, the pressure campaign seems to be working. Partisanship is a powerful drug, and when the heat is turned up, more often than not, politicians revert to the safe confines of their base for protection. This explains why McSally, a vulnerable Republican who can’t afford to be seen as a Trump quisling, is suddenly acting… like Trump !

It also explains why, presented with revelations of what might be rightly seen as blockbuster evidence that Marie Yovanovitch was under surveillance by Trump and Rudy Giuliani’s henchmen in Ukraine, moderate Maine Sen. Susan Collins’ first reaction was to... blame Congress!

Neither McSally nor Collins have, in the past, been considered particularly Trumpy. That’s why their defensive behavior is especially revealing. Despite the constant revelation of new, damning evidence as the impeachment trial kicks off, Republicans have cast their lot with this president and his base.

This was recently driven home to me by the analysis of a man I once considered to be a straight-shooting, center-right journalist. Though largely subsumed by an avalanche of news about Rudy Giuliani and Lev Parnas, the arrest of Michael Avenatti, and the final Democratic primary debate before the Iowa caucuses, a comment made by Fox News’ Brit Hume deserves more attention:

“Let's assume… just for the sake of discussion, that John Bolton comes in and he says, 'Yeah, the president wanted the Bidens investigated and he withheld the aid for a time to try to get that done.’ I don't think very many Republican senators are going to say that they think Trump did that or that he's guilty of that."

“ Martha McSally and Susan Collins are proof positive that even the “thoughtful” Republicans are so desperate to defend this president. ”

This, of course, was an amazing admission. Put aside the fact that Hume doesn’t seem outraged by any of this. His analysis (correct, I think), is that even if John Bolton (John Bolton!) testifies that Trump used the power and prestige of the presidency (not to mention our tax dollars) to extort the president of Ukraine into announcing an investigation into the Bidens, that most Republican senators wouldn’t believe it—or wouldn’t care.

Now ask yourself, how does that analysis comport with the oath that senators took on Thursday, which states that “in all things appertaining to the trial of the impeachment of Donald John Trump, president of the United States, now pending, I will do impartial justice according to the Constitution and laws: So help me God”?

Yeah...

I suppose it’s possible that at least four Republicans will, in fact, vote to allow witnesses, and that one of those witnesses will reveal something that is so explosive that 20 Republicans, having taken that oath, are forced to finally, reluctantly, cut Trump loose.

It just seems hard—almost impossible—to imagine what in the world could be so horrible. Martha McSally and Susan Collins are proof positive that even the “thoughtful” Republicans are so desperate to defend this president that they are taking a page out of his playbook of projection and prevarication.

Trump corrupts!