GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderOvernight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Overnight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Chris Murphy may oppose bipartisan health bill unless it addresses ObamaCare 'sabotage' 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 ClintonDemocrats bristle as Hicks appears for daylong Capitol Hill testimony Democrats bristle as Hicks appears for daylong Capitol Hill testimony Trump: 'So sad' Democrats are putting Hope Hicks 'through hell' 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 awards Presidential Medal of Freedom to economist, former Reagan adviser Arthur Laffer Trump awards Presidential Medal of Freedom to economist, former Reagan adviser Arthur Laffer Second ex-Senate staffer charged in aiding doxxing of GOP senators MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerPress: How 'Nervous Nancy' trumped Trump Press: How 'Nervous Nancy' trumped Trump Amash gets standing ovation at first town hall after calling for Trump's impeachment 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 McConnellOvernight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record Overnight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record Senate to vote Thursday to block Trump's Saudi arms deal MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerSenate confirms Trump judicial nominee criticized for being hostile to LGBT community Senate confirms Trump judicial nominee criticized for being hostile to LGBT community Democrats detail new strategy to pressure McConnell on election security bills 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 MurrayOvernight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Overnight Health Care: Trump officials defend changes to family planning program | Senators unveil bipartisan package on health costs | Democrats pass T spending bill with HHS funds Chris Murphy may oppose bipartisan health bill unless it addresses ObamaCare 'sabotage' 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 MarkeyHillicon Valley: Senate sets hearing on Facebook's cryptocurrency plans | FTC reportedly investigating YouTube over children's privacy | GOP senator riles tech with bill targeting liability shield | FAA pushed to approve drone deliveries Hillicon Valley: Senate sets hearing on Facebook's cryptocurrency plans | FTC reportedly investigating YouTube over children's privacy | GOP senator riles tech with bill targeting liability shield | FAA pushed to approve drone deliveries Overnight Defense: House passes T spending package with defense funds | Senate set to vote on blocking Saudi arms sales | UN nominee defends climate change record 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 GreenOvernight Energy: Trump appoints Social Security watchdog to also oversee Interior | Critics question EPA guidance on pipelines | Battle over science roils EPA Overnight Energy: Trump appoints Social Security watchdog to also oversee Interior | Critics question EPA guidance on pipelines | Battle over science roils EPA Trump appoints Social Security Administration watchdog to also oversee Interior 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 TrumpBooker hits Biden's defense of remarks about segregationist senators: 'He's better than this' Booker hits Biden's defense of remarks about segregationist senators: 'He's better than this' Trump says Democrats are handing out subpoenas 'like they're cookies' MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnYouTube may move children's content to separate app YouTube may move children's content to separate app Democrats detail new strategy to pressure McConnell on election security bills MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.