GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderLamar AlexanderAuthorities link ex-Tennessee governor to killing of Jimmy Hoffa associate The Republicans' deep dive into nativism Senate GOP faces retirement brain drain 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 ClintonBiden flexes presidential muscle on campaign trail with Virginia's McAuliffe Shontel Brown gaining ground against Nina Turner in Ohio: poll Biden hits trail for McAuliffe in test of his political brand 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 HatchDrug prices are declining amid inflation fears The national action imperative to achieve 30 by 30 Financial market transactions should not be taxed or restricted MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerCheney set to be face of anti-Trump GOP How leaving Afghanistan cancels our post-9/11 use of force The unflappable Liz Cheney: Why Trump Republicans have struggled to crush her  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 McConnellHouse Democrats grow frustrated as they feel ignored by Senate Democrats question GOP shift on vaccines Has Trump beaten the system? MORE (R-Ky.) and Minority Leader Charles SchumerChuck SchumerMcConnell pushes vaccines, but GOP muddles his message Biden administration stokes frustration over Canada Schumer blasts McCarthy for picking people who 'supported the big lie' for Jan. 6 panel 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 MurrayTech executives increased political donations amid lobbying push Schumer, Tim Scott lead as Senate fundraising pace heats up Sunday shows preview: As delta variant spreads, US leaders raise concerns 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 MarkeyEd MarkeyEquilibrium/ Sustainability — Presented by NextEra Energy — Olympics medals made of mashed up smartphones Lawmakers urge Biden to make 'bold decisions' in nuclear review OVERNIGHT ENERGY: Democrats lay out vision for Civilian Climate Corps | Manchin to back controversial public lands nominee | White House details environmental justice plan 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 Health Care: CDC encourages schools to open for in-person learning, masks optional | President directs moves on drug importation, calls for plan to lower drug prices | FDA asks for federal investigation of Alzheimer's drug approval Bipartisan lawmakers press NIH for info on deleted coronavirus data GOP rep demands Air Force Academy professor be removed for teaching critical race theory 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 TrumpTrump hails Arizona Senate for audit at Phoenix rally, slams governor Arkansas governor says it's 'disappointing' vaccinations have become 'political' Watch live: Trump attends rally in Phoenix MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnBiden's misinformation crackdown spotlights partisan divide on content reform White House looks to cool battle with Facebook Republicans raise concerns about Olympians using digital yuan during Beijing Games MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.