NASHVILLE, Tenn. >> Taking on a crowd of jeering union workers, standing up to a charismatic Democratic opponent on the man’s home turf or lecturing upper management of one of the world’s largest corporations, Tennessee Sen. Bob Corker has rarely backed down from a fight.

So it came as little surprise to those who know him well that the pugnacious self-made multimillionaire who once floated his own name as a 2016 Republican presidential candidate doubled down this week on his criticism of President Donald Trump, calling him “utterly untruthful” and responsible for “the debasement of our nation.”

“If he feels something isn’t right, he’s going to say that,” said Tennessee Gov. Bill Haslam, who has known Corker since he was a teenager.

Corker, 65, did, in fact, back down from a potentially serious primary fight last month when he announced he would not run for re-election, though he insisted he did so from a position of strength.

While he had clashed with Trump even before the announcement, it seemed to liberate him altogether to speak candidly about the president, which he did early this month, when he charged that Trump had turned the White House into an “adult day care center” and was setting the U.S. “on the path to World War III.”

Trump hit back at Corker as a “lightweight” and charged that the former Chattanooga mayor and two-term senator “couldn’t get elected dog catcher in Tennessee.” Trump insultingly dubbed him “Liddle Bob Corker.”

Jokes about Corker’s diminutive stature — he lists his height as 5-foot-7 — are hardly going to get a rise out of someone who has been hearing them at least since the former construction company owner first ran for the Senate in 1994.

That crowded primary was ultimately won by Bill Frist, a Nashville physician whose campaign labeled Corker a “desperate little man,” ”the Chattanooga boo-hoo” and “rotten pond scum.” Much of that vitriol stemmed from Corker’s pointing out Frist’s spotty voting record and calling him a cat killer for adopting felines and using them for experiments while he was a medical student.

Corker was next appointed state finance commissioner, where he broke with others in the GOP administration by supporting an ultimately successful effort by then-Nashville Mayor Phil Bredesen, a Democrat, to bring the NFL’s Houston Oilers to Tennessee. Bredesen was later elected governor and is now mulling a bid to succeed Corker in the Senate.

Bredesen, in announcing his interest in the race, lauded Corker’s service despite some differences over policy.

“I have long admired the way in which he puts the interests of Tennesseans and our nation ahead of political gamesmanship,” Bredesen said. “He educates himself on the issues and thinks independently about them, in exactly the way the founders of our nation intended.”

Following a single term as Chattanooga mayor, Corker made another run for the Senate in 2006 after Frist announced his retirement. Corker won by less than 3 percentage points, in part because of the fallout from Democrat Harold Ford Jr.’s attempts to confront him with the media in tow in Memphis, the politically powerful Ford family’s home base.

In what Corker’s campaign dubbed the “Memphis Meltdown,” the Republican met Ford head-on, telling him, “It’s a true sign of desperation that you would pull your bus up when I’m having a press conference.”

By 2014, he had risen to become chairman of the powerful Senate Finance Committee. He ended up sitting out the 2016 presidential race and refused to endorse any of the Republican candidates until Trump had won Tennessee’s Super Tuesday primary.

On a weekend mountain hike, Corker was confronted by Pulitzer Prize-winning author David George Haskell for supporting Trump. Corker challenged Haskell, a naturalized U.S. citizen from Britain, to explain what he has “ever done to contribute to the state,” adding: “If you don’t like it, leave,” according to Haskell.

The senator subsequently found himself in the Trump orbit, considered for both vice president and secretary of state but ending up with neither post. The two have different versions of who initiated those conversations.

Corker studied industrial management at the University of Tennessee in Knoxville, working in construction in the summer and after graduation and using the $8,000 he saved to start his own company with a single pickup truck. He hit it big when he won the contract to build drive-thru windows for the Krystal fast-food chain.

At a celebratory 2011 event to announce a return to production at a General Motors plant on the outskirts of Nashville, Corker was mercilessly heckled by hundreds of members of the United Auto Workers union who blamed the plant’s shutdown on his opposition to the 2009 auto-industry bailout.

Corker stared down the crowd and endured the jeers for 20 seconds.

“I see the saga continues,” he finally said, acknowledging past disagreements with the union. “I can tell today that that’s fine with you. And it’s fine with me.”

The senator later took a lead role in working to defeat the UAW in a union election at the Volkswagen plant in Chattanooga in 2014. And he warned Volkswagen managers in Germany that they would become a “laughingstock” if the union won the vote.

He appeared to revel in the union’s efforts to vilify him.

“I probably am Public Enemy No. 1 for the UAW,” he said with no hint of regret.