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 Donald Trump.

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

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“When I ran for the Senate in 2006, I told people that I couldn’t imagine serving for more than two terms,” said Corker, who chairs the Senate Foreign Relations Committee. “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 said.

Corker said serving the people of Tennessee as senator “has been the greatest privilege of my life” and that while traveling the state in August, “I was reminded that we live in a unique place full of people who care deeply about the direction of our country.”

Corker, a two-term senator and former Chattanooga mayor, 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.

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“He has gone through a process — his own process — of making a decision, and that has been going on for the last several weeks,” said his chief of staff, Todd Womack. “I think he always thought he would only serve two terms. His philosophy on public service is you get in, you serve and then you hand it off for somebody else to have an opportunity to serve. That has always been in the back of his mind.”

Corker reached his final decision Tuesday morning and informed Senate Majority Leader Mitch McConnell and his Tennessee colleague, Sen. Lamar Alexander. At 3:30 p.m, Corker broke the news to President Donald 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. Republicans including former state Rep. Joe Carr and state Sen. Mark Green also have said they might jump into the race, and others are likely to join them 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.

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“Even when he’s been investigating smugglers’ tunnels near the Gaza strip, talking to foreign leaders, or giving advice to President Trump, Bob has never let his feet leave the ground in Tennessee,” Alexander said.

“He says what he thinks, does what he believes is best for Tennesseans and has helped lead his colleagues on complicated issues involving the federal debt and national security. His absence will leave a big hole in the United States Senate, but I know he’s carefully weighed his decision, and I’m looking forward to seeing what he tackles next.”

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.”

Sen. Bob Menendez, the New Jersey Democrat who worked with Corker on both the Senate Foreign Relations Committee and the Senate Banking Committee, praised the Tennessee Republican’s willingness to reach across the aisle.

“With a keen intellect and deep understanding of the importance of diplomacy and promoting American values in addressing the challenges we face as a nation, Bob Corker’s unique brand of leadership will be sorely missed but not forgotten,” Menendez said.

Menendez called Corker “a powerful voice for reason, rationality, and common sense” and said his retirement “will leave a gaping void in the leadership of the Senate at a time when we need those qualities the most.”

In Tennessee, Gov. Bill Haslam said Corker “has served his city, our state and our country selflessly and with excellence.”

“He has made a positive difference in the lives of every Tennessean and every American,” Haslam said. “Bob has been a close friend for over 40 years. His leadership and wisdom in the Senate will be missed, but I have complete faith in his judgment and respect his decision. I look forward to seeing what he does next.”

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Tom Ingram, a Tennessee political operative who helped Corker with his 2006 Senate campaign, said he had been hoping Corker would decide to run for another term.

“Corker is unique and special in public service,” said Ingram, who first met Corker when the former Chattanooga mayor was serving as Tennessee’s Finance and Administration commissioner under Gov. Don Sundquist.

Corker “doesn’t make decisions about running the way so many do, who make it because it’s the next thing to do,” Ingram said. “He really thinks seriously about it, thinks about how he’s going to be using his time, where and how he can make the greatest difference.”

In Washington, Corker went from “being kind of a rogue cynic about the whole process and environment to being someone who became an effective part of the environment but without giving up his independence,” Ingram said.

“You could always count on Corker to make up his own mind,” he said. “He would try to be a team player, but if he couldn’t be a team player he was going to be himself at all times.”

"He had that kind of respect, even early on in the Senate. And he’s continued to have the distinguished curiosity and intelligent approach to the foreign relations committee.”

Corker’s announcement that he won't run for another term follows his unusual — and very public — airing of grievances with Trump earlier this summer.

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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.

Corker, who earlier had been considered as Trump’s vice presidential 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 asa productive discussionon a broad range of foreign and domestic issues. At that meeting, Trump encouraged Corker to run for another term.

Corker was first elected to the Senate in 2006, defeating Democrat Harold Ford Jr. of Memphis 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.

Reporter Dave Boucher in Nashville and Herb Jackson of USA TODAY contributed to this story.