GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar Alexander Embattled senators fill coffers ahead of 2020 GOP senators divided on Trump trade pushback Five things to know about the measles outbreak 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 ClintonThorny part of obstruction of justice is proving intent, that's a job for Congress Nadler: I don't understand why Mueller didn't charge Donald Trump Jr., others in Trump Tower meeting Kellyanne Conway: Mueller didn't need to use the word 'exoneration' in report 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 HatchHatch warns 'dangerous' idea of court packing could hurt religious liberty Former Democratic aide pleads guilty to doxing GOP senators attending Kavanaugh hearing How do we prevent viral live streaming of New Zealand-style violence? MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerPollster says Trump unlikely to face 'significant' primary challenge GOP gets used to saying 'no' to Trump Democrats introduce bill to rein in Trump on tariffs 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 McConnellSenate Republicans tested on Trump support after Mueller Anti-smoking advocates question industry motives for backing higher purchasing age Former Bush assistant: Mueller report makes Obama look 'just plain bad' MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerHillicon Valley: House Dems subpoena full Mueller report | DOJ pushes back at 'premature' subpoena | Dems reject offer to view report with fewer redactions | Trump camp runs Facebook ads about Mueller report | Uber gets B for self-driving cars Dem legal analyst says media 'overplayed' hand in Mueller coverage Former FBI official praises Barr for 'professional' press conference 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 MurrayHillicon Valley: Washington preps for Mueller report | Barr to hold Thursday presser | Lawmakers dive into AI ethics | FCC chair moves to block China Mobile | Dem bill targets 'digital divide' | Microsoft denies request for facial recognition tech Dems introduce bill to tackle 'digital divide' Only four Dem senators have endorsed 2020 candidates 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 MarkeyTlaib rallies in support of Green New Deal at Detroit town hall Ben & Jerry's backs Green New Deal: 'We have to act now' Warren praises Ocasio-Cortez in Time 100 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 GreenRepublicans call for ex-Trump lawyer Cohen to be referred to DOJ Finally united, House Republicans refer ex-Trump lawyer Michael Cohen for perjury prosecution Overnight Defense: Trump labels elite Iranian military unit a terrorist group | Iran hits back with terrorist label for US Central Command | US troops, contractor killed in Afghanistan blast 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 TrumpThorny part of obstruction of justice is proving intent, that's a job for Congress Obama condemns attacks in Sri Lanka as 'an attack on humanity' Schiff rips Conway's 'display of alternative facts' on Russian election interference MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnConservative groups defend tech from GOP crackdown Lawmakers weigh challenges in fighting robocalls Senators show deep skepticism on Space Force proposal MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.