Six days later, precisely zero Republican senators have added their names to Corker’s criticisms. Why?

Washington Post columnist Michael Gerson has a theory: Cowardice. “More than anything else at this moment,” he writes, “the nation has need of Republican vertebrates.”

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But politicians aren’t just reflexive cowards; they are often calculated ones. And there are plenty of reasons we haven’t seen a mass exodus of GOP lawmakers from Trump — if lots of them do indeed agree with Corker.

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Here are five non-mutually exclusive possibilities:

1. They still hope — against hope — that Trump will change

The media isn’t the only group that seems to be constantly waiting in vain for Trump to change. Surely he has to get that this isn’t working, they think. Maybe this speech suggests he’s toning it down, they think. John Kelly will restore some order, they think.

When you don’t understand something, it’s easy to think it will eventually correct its course into something more familiar — more tried and true. And just like some journalists seem to be half expecting Trump will eventually get it, Republican members of Congress probably half expect he will as well. In fact, they’re probably more likely to believe he will, because they really want him to.

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Speaking out against Trump en masse is the point of no return — the moment they give up on everything and invite open warfare with a president from their own party. And you don't go to war until you’re sure diplomacy can’t work.

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2. It’s an unnecessary risk, from a self-preservation standpoint

If there’s one thing politics selects for, it’s the self-preservation gene. The ditches along the road to power are littered with the corpses of the idealistic and the uncompromising. These members — especially those in the Senate — didn’t get here by introducing unnecessary risks into their careers.

And from strictly a self-preservation standpoint, speaking out against Trump is an unnecessary risk. It’s no surprise that the most vocal senator to speak out against him happens to be the only senator who has announced he is retiring in 2018. Most GOP senators, by contrast, will have to seek reelection in clearly red states — 20 states went for Trump by double-digits — where the general election is a cinch. That means their only obstacle to reelection is inviting a primary challenger or irritating the groups that might fund that primary challenger.

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Even if speaking out against Trump is safe — and there’s evidence that it’s not, which we’ll get to — why even stir the pot? You’ve got lots of important things to do, after all, and you can only do them if you’re still a U.S. senator!

3. Some have paid the price for it

There is plenty of evidence that those who pit themselves against Trump suffer for it — especially in their standing with the GOP base.

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We have limited polling on all senators, but the polling we do have all points in the same direction: that it’s not just an unnecessary risk but a real hazard to reelection.

4. They don’t trust themselves

Trump was supposed to blow over. He was never supposed to run. Then he wasn’t supposed to win the primary. Then he wasn’t supposed to win the presidency.

The president has overcome the smart-people doubters at every turn. And it has to have congressional Republicans worried about underestimating him again. What if they simply don’t understand his appeal and his political instincts? What if Trump is playing the long game, and somehow they aren’t sophisticated enough to understand it?

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They may not believe that, but you have to believe that there’s at least 10 percent of their mind that isn’t certain. And that’s enough to stymie any action. Trump has a way of taking what you see with your very own eyes and making you question whether it’s real.

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5. It could destroy the GOP

Feuding with Trump isn’t just throwing in the towel on the GOP agenda for the remainder of his presidency and possibly on your reelection hopes; it would also threaten to tear the party in half, setting off an existential crisis in which the GOP base is pitted against the establishment. These tensions have been around for the better part of a decade, ever since the tea party came along, and a full-scale war could mortally wound the GOP at a time when it’s actually about as powerful as ever.

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Even if Trump is a nuisance whom Republicans simply can’t stand and they’d love to speak out against him, is it worth tearing the party apart to do so? If you’re a Democrat, you look at that last sentence and look at what Corker said, and you are apoplectic. Of course it's worth it. But if you’re not so certain that Trump is that existential threat, if you’re ideologically aligned with him and you think that this whole thing might eventually blow over, you may come to a different conclusion.