After roughly a quarter century in elected office, U.S. Sen. Lamar Alexander will retire, marking an end to one of the most remarkable political careers in Tennessee history.

The former Republican governor, who has served in the Senate since first being elected in 2002, said Monday that he will not seek a fourth term in the upper chamber.

“I will not be a candidate for re-election to the United State Senate in 2020,” Alexander said in a statement announcing his decision.

He thanked the people of Tennessee, calling them generous.

"I am deeply grateful, but now it is time for someone else to have that privilege," said Alexander, who has served in roles ranging from governor and U.S. senator to leading the U.S. Department of Education and the University of Tennessee.

Alexander's decision means that for the second time in two years, Tennessee will have an open U.S. Senate race.

Like U.S. Sen. Bob Corker, who last year declined to run for a third term, Alexander’s announcement will send shockwaves throughout Tennessee’s political landscape.

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Decision reached during August fishing trip

In an interview inside his Nashville office, Alexander said he decided not to run for another term during a fishing trip at Turtle River Camp in Bending Lake, Ontario.

"I decided in August and I didn't tell anybody," he said. "I decided then that this was enough and it was time to bring it to an end, hopefully on a good note."

Alexander said he didn't want to tell anyone to "let it simmer" and make sure he was making the right decision.

"I admire people who know when to leave, and I think it's better to leave when you're hitting .400 than some other time," he said, making a baseball analogy.

Alexander said he informed President Donald Trump about his decision on Sunday afternoon.

At one point in the call, Trump said he wanted to talk about Alexander serving in the Senate for the next 20 years.

"I said 'Mr. President, I'm calling to tell you I'm not going to run for re-election,'" said Alexander, who shared the news with his staff Monday morning.

Trump vowed to find ways to work with Alexander during the remainder of his term.

Contenders will line up for 2020 campaign

Although it is not clear who may vie for the seat, contenders could include Gov. Bill Haslam and incoming U.S. Rep. Mark Green, R-Clarksville.

Haslam flirted with the idea of running for the Senate when Corker announced his retirement and said Monday he is considering a 2020 bid. Green has expressed interest in running for the seat.

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Others who could consider a bid include U.S. Rep. Diane Black, who unsuccessfully ran for the Republican nomination for governor earlier this year.

Bill Hagerty, the current U.S. ambassador to Japan and former commissioner of the Tennessee Department of Economic and Community Development, could also throw his name into the campaign, as well as Nashville physician Manny Sethi.

Alexander said although he learned a long time ago that he wasn't elected to tell people how to vote, he said Haslam and Hagerty were the "most obvious" candidates for the Senate.

"Senate seats don't come up very often and I imagine it will attract a large number of candidates, and I think I'll let the people sort that out," he said.

Among Democrats, the list of potential candidates could include Chattanooga Mayor Andy Berke, state Sens. Jeff Yarbro of Nashville and Sara Kyle of Memphis, state Rep. John Ray Clemmons of Nashville and Iraq War veteran James Mackler.

Mackler entered the 2018 U.S. Senate race but ultimately dropped out when former Gov. Phil Bredesen launched his own campaign.

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Alexander remains popular

To some, Alexander’s decision may come as a surprise. In recent years, he has been a continuing presence in Washington, D.C., sponsoring measures related to the music industry, education and opioids that have become law.

Alexander is chairman of the key Senate Committee on Health, Education, Labor, and Pensions, which handles everything from education policy to issues with the Affordable Care Act.

He helped shepherd through a major overhaul of federal education law with the Every Student Succeeds Act, but also faced intense scrutiny and criticism for his support of some Trump administration actions — including helping push through the nomination of Betsy DeVos as education secretary.

But in many ways, his retirement makes sense. Alexander would be 86 years old by the end of a fourth term.

And although he’s been supportive of Trump, Alexander could face a Republican challenger who is further to the right on the political spectrum, making a re-election bid more difficult.

When he last ran for re-election in 2014, Joe Carr, a tea party aligned Republican, lost by just 9 points, leading some political observers to see Alexander as vulnerable.

On Monday, Alexander said he was not afraid about the prospect of facing a potential primary in 2020.

"I've been in a lot of campaigns and have a pretty good record," he said, noting he has never lost a primary election dating back to 1974. "I knew that if I ran for re-election it would be like starting over again."

Alexander said he thought he would have been in good shape in a Republican primary election if he were to run again. A recent poll found Alexander remained popular among likely Republican primary voters.

Alexander has storied Tennessee political career

Born in Maryville, Alexander received a bachelor’s degree from Vanderbilt University and a law degree from New York University.

In 1965, he served as a law clerk and messenger for John Minor Wisdom, a federal judge with the Fifth Circuit U.S. Court of Appeals. In the late 1960s, Alexander served as assistants to then-U.S. Sen. Howard Baker Jr., and President Richard Nixon.

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Alexander first ran for governor in 1974 but lost to Democrat Ray Blanton — making it Alexander's only general election defeat in his career. Then in 1978, Alexander ran for governor again, this time famously walking across the state in his successful bid.

Since then, he’s served two terms as governor, three years as president of the University of Tennessee, nearly two years as U.S. Secretary of Education under the late President George H.W. Bush — all before his 2002 election to the U.S. Senate.

He also made unsuccessful presidential runs in 1996 and 2000, dropping out of the race both times.

By the time he departs from the upper chamber, Alexander will have spent more combined years as governor and U.S. Senator than any other Tennessean in history.

John Geer, a Vanderbilt University political science professor and co-director of the university's poll, said Alexander’s pending departure from the Senate means the state is losing someone in Washington with experience, clout, and seniority.

“There aren’t enough people like him in the Senate these days,” Geer said. “That’s the truth of the matter, and that’s the sad state of things. You want to have people who are committed to certain views, which is important, but who are also willing to work across the aisle when the need arises.”

'The legislator of the decade'

After Alexander announced his decision, Alexander received praise from elected officials from across Tennessee and the nation, including Democrats.

Senate Majority Leader Mitch McConnell, R-Ky., called Alexander one of the "most consequential Senators on domestic policy in memory" and said the Senate will be "lesser without Lamar’s wisdom, collegiality, and expertise."

"For his colleagues in both parties, he is a go-to expert on some of the most critical subjects that directly impact Americans’ lives — particularly American families’ healthcare and our children’s schools," McConnell said.

U.S. Sen. Tim Kaine, D-Va., with whom Alexander has played piano with, said the Tennessean cares "passionately about the institution of the Senate."

And Corker, who since his 2006 election has served alongside Alexander, praised his colleague for his commitment to the Senate and state.

“I often tell him he is the legislator of the decade because of the effective way he has worked across the aisle to pass legislation that directly affects the lives of so many throughout our state and around the country," Corker said.

He called Alexander one of the finest statesmen the state has ever seen.

"I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said.

Haslam said it was "almost impossible to measure the impact" of Alexander's commitment to Tennessee, noting his work to contribute to the state's economic position and eventual education reforms.

"As a senator, he has distinguished himself as a national leader, while always reminding everyone that our founders designed our government for most of the power to be delegated to the states," Haslam said in a statement. "No one has served our state longer as a governor and senator, and few, if any, have served it better than Lamar.”

Lt. Gov. Randy McNally, R-Oak Ridge, who was a young legislator while Alexander served as governor, described Alexander as "a Tennessee legend."

Along with Baker and former Gov. Winfield Dunn, Alexander is the one "responsible for the modern rebirth of the Republican Party," McNally said in a statement,

"But he always put principles over party, people over politics," McNally said. "Lamar always did what was best for Tennessee."

Natalie Allison contributed to this report.

Reach Joel Ebert at jebert@tennessean.com or 615-772-1681 and on Twitter @joelebert29.