Sen. Bob Corker (R-Tenn.) speaks to journalists before Senate party caucus luncheons on Capitol Hill in Washington Tuesday October 31, 2017. (Melina Mara/The Washington Post)

Sen. Bob Corker was on his way to the Senate chamber for a vote, drinking coffee from a foam cup — and resolutely mum when asked about President Trump’s upcoming trip to Asia, his tax-reform strategy and what the Tennessee Republican meant when he called the White House an “adult day-care center.”

“I have no desire to enter into, you know, 24/7, you know, disagreement,” Corker explained in a brief interview. “When I have strong disagreements, I’m going to express them strongly.”

Corker seized the role of presidential critic in chief last month, when he accused Trump, in a rapid-fire series of conversations with reporters, of "debasing" the country with his "untruths" and "name-calling." If he could do it all over, he added, he would not have supported Trump in the 2016 presidential election.

It was a deeply personal and seemingly calculated fusillade — and the culmination of an uneven year in which Corker tried to partner with a new Republican White House but ultimately couldn’t resist the urge to very publicly walk away.

“The two things he cannot stand,” said Corker’s longtime chief of staff, Todd Womack, “are bullies and people who do not tell the truth.”

Now, Corker has his party on edge, wondering where and when he will strike again and what it will mean — for the fragile GOP alliance, for the party’s languishing agenda and for Corker himself, as he gears up for a final year in the Senate before retirement.

“I worry about what it does to his ability to be effective with the White House, with his peers,” said Tom Ingram, Corker’s longtime friend and former political strategist.

No one, not even Corker, seems sure where this one will lead.

Asked whether key moments changed his thinking about Trump, Corker demurred — promising to explain it all eventually, but not yet.

“The answer to that is very specifically, absolutely, yes,” he said. “But I don’t want to rehash those right now. I will at some point, probably.”

Interviews with Corker’s associates, friends, colleagues and aides revealed some of those turning points, including concern over the summer about Trump’s fiscal ideas and later, Trump’s controversial assessment of the deadly violence in Charlottesville.

These associates also explained that Corker’s frustration built over time, and that his outbursts of frustration combined impulse, circumstance and the kind of gut-level decision-making that has guided a lifetime of unconventional choices.

Unlike Sen. Jeff Flake (R-Ariz.), who has harshly and consistently criticized Trump since the 2016 campaign, Corker slowly lost faith, both with the president’s disposition as a leader and with the policies his White House was pursuing.

A construction tycoon, Corker forged an early bond with Trump over their shared business backgrounds. His name surfaced as a potential running mate and later for secretary of state.

He used such words as "incredible" and "excited" and "positive" to describe Trump. They played golf and talked on the phone.

Republican presidential candidate Donald Trump appears with Sen. Bob Corker R-Tenn.) during a campaign event at the Duke Energy Center for the Performing Arts in Raleigh, N.C., last July. (Sara D. Davis/Getty Images)

As chairman of the Foreign Relations Committee, Corker became known for an affable role giving the president foreign-policy advice. According to Ingram, Corker had felt “some disappointment” that he wasn’t tapped for a high-level administration post. But “he moved on pretty quickly.”

Evidence of strain emerged over the spring and summer. After the revelation in May that Trump had disclosed classified information to Russia's foreign minister and ambassador, Corker said that the White House was in a "downward spiral."

A couple of months later, Corker voiced private concern with his own political strategists that Trump was not doing enough to curb federal spending, according to two people familiar with the conversations.

And just days after Trump stoked controversy in blaming "both sides" for what happened at a deadly white-supremacist rally in Charlottesville, Corker said Aug. 17 that Trump had "not yet been able to demonstrate the stability nor some of the competence that he needed to demonstrate in order to be successful."

But it wasn’t just the Charlottesville comments that troubled Corker, people close to him said. Other controversial actions Trump had taken also weighed on his mind. The setting that day also contributed to his decision to speak out. He was asked about Trump by reporters at the Rotary Club in his home town of Chattanooga, where he had served as mayor. He wanted to be upfront with the folks back home.

Corker was also struck with the growing realization that what concerned him about Trump was not getting better, associates said. He had talked to Trump over dinners and other private chats. He had expressed his worries to confidants in less public settings. His hope had been that Trump would change his ways. That didn’t happen.

On Sept. 26, Corker announced he would not run for reelection. His decision came as a surprise even to some people in his inner circle. He had been preparing for a campaign — traveling back home, raising money.

Womack said two days shaped Corker’s decision more than any others. One was an economic development announcement on Aug. 24 in Middle Tennessee. While sitting on stage and looking at the audience, he reflected on his time in the Senate and felt a sense of completion. This is where he knew in his heart it was time to step down, Womack said.

The second was at a fundraiser in Washington the night before the announcement, where he voiced pessimism about the Republican legislative agenda. He knew in his head that moment, Womack said, that it was time to bow out.

Others in Corker’s orbit believe the prospect of a difficult Republican primary also weighed on the senator’s mind — a primary that grew likelier as his feud with Trump escalated.

Trump has fueled the conflict, too. He has disparaged the 5-foot, 7-inch senator as "Liddle' Bob Corker" on Twitter and branded him an ineffective senator.

Trump's contradictions and criticism of Secretary of State Rex Tillerson also bothered Corker. On Oct. 4, Corker told reporters, "I think sometimes people stay in incredibly frustrating positions like that because they feel like they know what it is that they're — you know — the kind of policies that they are putting in place."

Secretary of State Rex Tillerson, left, Sen. Bob Corker R-Tenn.), center, and Defense Secretary Jim Mattis enter a Senate Foreign Relations Committee last week. Corker serves as the committee’s chairman. (Keith Lane/Getty Images)

For Corker, impulses and uncertainty have governed a lifetime of audacious moves. According to associates, he has always gone his own way, eschewed advice and taken risks — without always knowing how things will end.

There was the time he joined his high school football team as a defensive back.

"I was 5'6" and 140 pounds, and at a rivalry game, a very large running back — probably 200 pounds — was coming down the sideline at me full speed," Corker once told a local publication. "I caught him square in the chest and drove him backwards. I remember Coach Davis grabbing me by the back of my pants in excitement after the play. I was proud because I had executed it in the exact way I had been taught."

Decades later, Corker broke with party leaders to join Democrats in the Dodd-Frank financial regulation talks during Barack Obama’s presidency, only to be left empty-handed at the conclusion.

Some Republicans have been relieved that Corker has stayed largely silent since he went off on the president Oct. 24. He had chosen to speak out at a moment when Republicans were supposed to be fostering unity and cohesion, not deepening fissures. That very day, Trump was in the Capitol to visit with Senate Republicans and gin up support for the tax overhaul.

“There’s a biblical concept of Matthew, Chapter 18 — when you have a dispute with somebody, you go to them face to face and be able to work it out,” said Sen. James Lankford (R-Okla.) on the day of Corker’s criticism. “And it’s still not a bad practice no matter where you are.”

Most Republican senators have avoided public comment on the Trump-Corker fight. Privately, some have encouraged peace.

Senate Foreign Relations Committee Chairman Bob Corker R-Tenn.) talks to reporters as he heads to the floor for a vote last month. (J. Scott Applewhite/AP)

“We’ve all visited,” said Sen. Mike Rounds (R-S.D.), when asked if he has spoken to either Corker or Flake, who announced his own retirement with a long rebuke of the president on the same day that Corker spoke out. “And we all understand some of their frustrations. That’s part of it. Look, these are two good people. But Donald Trump is the president of the United States. And for the good of this entire country, we want him to be successful.”

On his way to the Senate vote last week, Corker said he had not spoken to the president since his denunciation the week before. Womack attributed Corker’s desire not to revisit his criticism of Trump as part of his goal of being productive and focusing on policy.

Soon, however, Corker and Trump would be at odds again.

A day later, on Friday, Corker fired off a statement criticizing Trump for demanding that the Justice Department and FBI target Democrats and for calling for the death penalty for the suspect in the New York City terrorist attack.