In fairness, Trump has always had the upper hand in this relationship. When he was sailing to the GOP nomination, Republican lawmakers were almost universally reluctant. But even those who said he wasn't qualified for the presidency eventually rallied to him in the name of getting a (nominal) Republican elected.

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Now that he's president, Trump holds the keys to the GOP majorities in the House and Senate actually accomplishing something with their newfound power. Along that bumpy road, lawmakers including House Speaker Paul D. Ryan (R-Wis.) and Senate Majority Leader Mitch McConnell (R-Ky.) have taken verbal abuse from Trump and, at times, seen him railroad their entire agenda. Their response has almost always been muted as they wait for the latest fury to blow over.

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We've seen it with Trump aides, too. Trump has virtually tortured some of them, from Sean Spicer to Reince Priebus even to Stephen K. Bannon and now John Kelly. He's publicly derided and/or contradicted his secretary of state, his attorney general and his now-former health secretary. It's been reported that many of those around him are staying there simply to avert the kind of Trumpian chaos that Corker referenced. The Trump administration is truly where you pride goes to die.

That's a terrible arrangement for everyone involved. But it's only terrible as long as Republicans are unwilling to stand up to Trump publicly. As plenty of reporters have noted, the things Corker is now saying are what many aides and lawmakers say off the record or on-background. Corker (and, increasingly, a few others) are only attaching their names to the criticisms of Trump that have long been obscured by the veil of anonymity.

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Lawfare's Susan Hennessey sums it up well:

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And their reasons are obvious: They still want to get things done. They're not ready to throw in the towel and make an enemy of the president. But whether it's “cowardice, dumbfounded inaction or — what I would argue — a temptation to gloss over the dire situation they're in, their combined reluctance is only reinforcing the upper hand they've given Trump. They have power in this relationship, too, because they hold the keys to Trump actually passing his agenda through Congress. Trump can't just go through Democrats and hope to please his base, as he's showing on immigration.

Politics is a business that rewards risk mitigation. Most lawmakers represent safe districts and states and only have to worry about tempting primary challengers. Thus, running afoul of Trump is an unnecessary risk. It's no coincidence that the most strident criticisms of Trump are now coming from a GOP senator who recently announced his retirement.

But until Trump's comments about Corker are viewed is crossing a line in the sand — until lawmakers treat Trump's broadsides against them as a point of no return when it comes to having their support — he has little reason to stop it. If he thinks he can just bring you back into the fold with the temptation of tax reform, it's never going to stop. If he can attack you and your family and then break bread with you on legislation, he has no reason to stop.