WASHINGTON — Sen. Bob Corker announced Tuesday he will not seek re-election next year, bringing to a close the Senate career of an influential Republican who has been a key player on foreign policy and both a staunch defender and critic of President Trump.

“After much thought, consideration and family discussion over the past year, (wife) Elizabeth and I have decided that I will leave the United States Senate when my term expires at the end of 2018,” the Tennessee Republican said in a statement.

“When I ran for the Senate in 2006, I told people that I couldn’t imagine serving for more than two terms. Understandably, as we have gained influence, that decision has become more difficult. But I have always been drawn to the citizen legislator model, and while I realize it is not for everyone, I believe with the kind of service I provide, it is the right one for me.

“I also believe the most important public service I have to offer our country could well occur over the next 15 months, and I want to be able to do that as thoughtfully and independently as I did the first 10 years and nine months of my Senate career."

Corker, a two-term senator who chairs the influential Senate Foreign Relations Committee, had agonized for weeks over whether he should run for another term amid speculation that he would likely face a challenge from the GOP’s right flank.

He reached his final decision Tuesday morning and informed Senate Majority Leader Mitch McConnell, R-Ky., and his Tennessee colleague, Sen. Lamar Alexander. At 3:30 p.m., Corker broke the news to President Trump during a phone call while Trump was traveling aboard Air Force One.

Corker’s announcement immediately set off speculation about who would replace him in what will likely turn out to be Tennessee’s marquee race next year.

Conservative activist Andy Ogles already has announced his intentions to run for the seat. Other Republicans, including former state representative Joe Carr and state Sen. Mark Green, also have said they are may jump into the race. Others will likely run now that Corker is out of the picture.

On the Democratic side, Nashville attorney and Iraq War veteran James Mackler already has announced plans to run.

The news that Corker will retire brought a flood of congratulations — and expressions of remorse — from his Senate colleagues.

McConnell said Corker “has been an integral member of our team and confidant of mine during his time in the Senate.”

“His leadership on important issues has helped guide our conference and had a real impact at home and abroad,” McConnell said. “I know he looks forward to being a part of the important agenda items we have in front of us for the remainder of this Congress.”

Senate Democratic leader Chuck Schumer of New York called Corker “a friend and a fine, conscientious, and hard-working senator. His thoughtfulness and dedication to the job make him a model senator. We all regret him leaving.”

Corker's announcement follows his unusual — and very public — airing of grievances with Trump earlier this summer.

The two traded jabs in August after Corker, following Trump’s much-criticized response to the deadly violence in Charlottesville, Va., told reporters that Trump had not yet shown the stability nor some of the competence needed to become successful.

A week later, Trump fired back at Corker on Twitter, calling Corker’s statement strange “considering that he is constantly asking me whether or not he should run again in '18.”

“Tennessee not happy!” Trump exclaimed.

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Corker, who earlier had been considered as Trump’s running mate and secretary of State, insisted his relationship with Trump had not been harmed by the exchange.

A little more than a week ago, the two met for more than an hour in the Oval Office in what spokesmen for both men described as a productive discussion on a broad range of foreign and domestic issues. At that meeting, Trump encouraged Corker to run for another term.

Corker, a former Chattanooga mayor, was first elected to the Senate in 2006, defeating Democrat Harold Ford Jr. in an otherwise difficult election season for the GOP. Republicans lost their majorities in both the Senate and the House that year. Corker was the only Republican newcomer to win a Senate seat that year.

In the Senate, Corker quickly developed a reputation as lawmaker who would side with then-president George W. Bush on some issues but challenge him on others. For example, Corker backed an energy bill that his fellow Republicans tried to kill. He also went against the White House and supported the expansion of a health insurance program for children.

Corker was easily re-elected in 2012 against Democrat Mark Clayton, an anti-gay activist denounced by his own party.

When Republicans reclaimed their Senate majority two years later, Corker became chairman of the Senate Foreign Relations Committee, a job that elevated his profile on the national stage and put him at the center of national and international issues.