By May, Corker was rumored to be a leading contender to join Trump as a vice-presidential candidate. But he still had reservations. After Trump attacked Judge Gonzalo Curiel as unfit to preside over a Trump University fraud case because of his Mexican ancestry, the senator was critical.

“He’s obviously stepped in it. He’s made statements that are inappropriate,” he said in June. “He’s got this defining period that’s over the next two or three weeks where he could pivot, can pivot, hopefully will pivot to a place where he becomes a true general election candidate.”

How brave you found this depended on what you thought Corker’s ambitions were. If he wanted to be vice president, this was some tough speech, but then again Corker was also simply saying what many Republicans, whether they supported Trump or not, believed: The campaign was on the verge of collapse.

Apparently Trump passed the three-week test (though he neither withdrew the Curiel attack nor notably moderated his tone and language), because a month later Corker was stumping for him in North Carolina, though he said on July 6 that he was withdrawing from the veepstakes.

Later that month, Trump was officially nominated for president, and most of the GOP rallied around him, as Corker had long before. When a tape emerged of Trump boasting about sexually assaulting women, Corker was critical—“These comments are obviously very inappropriate and offensive and his apology was absolutely necessary”—but did not step down from an advisory position to which he’d been appointed that day.

That looked like a wise (if ethically arguable) gamble after Trump won the race, surprising even himself, and Corker became a frontrunner to be secretary of state. Although many Republicans were eager for Corker to get the job—seeing him as a sober adult who would keep Trump in line—the senator said he doubted he’d get the pick, and in fact the job went to Rex Tillerson, who has endured a painful and ineffectual tenure that may end soon. Long-escalating tension between the president and his secretary of state has only intensified in recent weeks.

Corker has seemed to get more and more disillusioned about Trump as time goes on. In August, after Trump endorsed a soft white supremacy in the wake of the race riot in Charlottesville, Corker criticized him harshly.

“The president has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful,” Corker told Chattanooga website Nooga.com. “He has not demonstrated that he understands what has made this nation great and what it is today, and he's got to demonstrate the characteristics of a president who understands that.”

Trump fired a warning shot shortly after that:

Strange statement by Bob Corker considering that he is constantly asking me whether or not he should run again in '18. Tennessee not happy! — Donald J. Trump (@realDonaldTrump) August 25, 2017

Whether or not that was true, Corker was indeed weighing running for reelection. On September 26, he announced he would retire when his term ends next year. That day, he made a statement that sounded like a warning to the Trump administration. “I also believe the most important public service I have to offer our country could well occur over the next 15 months,” he said.