GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderRepublicans considering an outdoor stadium for Florida convention: report Sixth GOP senator unlikely to attend Republican convention Coronavirus Report: The Hill's Steve Clemons interviews Randi Weingarten 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 ClintonTrump confirms 2018 US cyberattack on Russian troll farm Hillicon Valley: Facebook considers political ad ban | Senators raise concerns over civil rights audit | Amazon reverses on telling workers to delete TikTok Biden campaign hires top cybersecurity officials to defend against threats 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 HatchMellman: Roberts rescues the right? DACA remains in place, but Dreamers still in limbo Bottom line MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerCheney clashes with Trump Sessions-Tuberville Senate runoff heats up in Alabama GOP lawmakers stick to Trump amid new criticism 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 McConnellHillicon Valley: Facebook considers political ad ban | Senators raise concerns over civil rights audit | Amazon reverses on telling workers to delete TikTok Ernst: Renaming Confederate bases is the 'right thing to do' despite 'heck' from GOP Advocacy groups pressure Senate to reconvene and boost election funding MORE (R-Ky.) and Minority Leader Charles SchumerChuck SchumerDemocrats blast Trump for commuting Roger Stone: 'The most corrupt president in history' A renewed emphasis on research and development funding is needed from the government Data shows seven Senate Democrats have majority non-white staffs 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 MurrayGOP Health Committee chair says he disagrees with Trump's WHO decision Lobbying battle brewing over access to COVID-19 vaccine Trump officials seek to reassure public about safety of a potential coronavirus vaccine 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 MarkeyGOP senators debate replacing Columbus Day with Juneteenth as a federal holiday The Hill's Campaign Report: Jacksonville mandates face coverings as GOP convention approaches Steyer endorses Markey in Massachusetts Senate primary 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 GreenComer tapped to serve as top Republican on House Oversight Clyburn pledges not to recognize committee members who don't wear masks This week: Democrats set to move health care, infrastructure proposals with eye on November 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 TrumpDemocrats blast Trump for commuting Roger Stone: 'The most corrupt president in history' Trump confirms 2018 US cyberattack on Russian troll farm Trump tweets his support for Goya Foods amid boycott MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnKoch-backed group urges Senate to oppose 'bailouts' of states in new ads The Hill's Coronavirus Report: Teachers' union President Randi Weingarten calls Trump administration plan to reopen schools 'a train wreck'; US surpasses 3 million COVID-19 cases The Hill's Coronavirus Report: DC's Bowser says protesters and nation were 'assaulted' in front of Lafayette Square last month; Brazil's Bolsonaro, noted virus skeptic, tests positive for COVID-19 MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.