GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderGOP braces for impeachment brawl McConnell tightlipped as impeachment furor grows GOP senator: 'Inappropriate' to discuss opponents, but impeachment a 'mistake' MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonWarren defends, Buttigieg attacks in debate that shrank the field Democrats fear Ohio slipping further away in 2020 Poll: Warren leads Biden in Maine by 12 points MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchTrump holds more Medal of Freedom ceremonies than predecessors but awards fewer medals Trump to award Medal of Freedom to former Attorney General Edwin Meese Trump to award racing legend Roger Penske with Presidential Medal of Freedom MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerTrump's GOP impeachment firewall holds strong George Conway hits Republicans for not saying Trump's name while criticizing policy Trump announces, endorses ambassador to Japan's Tennessee Senate bid MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. 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 in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellTrump-GOP tensions over Syria show signs of easing Trump again vetoes resolution blocking national emergency for border wall Trump invites congressional leaders to meeting on Turkey MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerTrump defends 'crime buster' Giuliani amid reported probe Louisiana voters head to the polls in governor's race as Trump urges GOP support Trump urges Louisiana voters to back GOP in governor's race then 'enjoy the game' MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayDemocrats urge Rick Perry not to roll back lightbulb efficiency rules Biz groups say Warren labor plan would be disaster Freedom of the press under fire in Colorado MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeySanders: 'Damn right we will' have a job for every American Democrats urge Rick Perry not to roll back lightbulb efficiency rules Ocasio-Cortez taps supporters for donations as former primary opponent pitches for Kennedy MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenInterior gains new watchdog We need a new structure to secure our border Tackling China in modern Cold War MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpWarren defends, Buttigieg attacks in debate that shrank the field Five takeaways from the Democratic debate in Ohio Democrats debate in Ohio: Who came out on top? MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnTrump-GOP tensions over Syria show signs of easing Congress set for showdown with Trump over Kurds GOP requests update on criminal referrals prompted by 2018 Kavanaugh probe MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.