GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderCongress must move forward on measure dealing with fentanyl GOP advances rules change to speed up confirmation of Trump nominees Key doctors group faces political risks on guns 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 ClintonOvernight Defense: Trump declares border emergency | .6B in military construction funds to be used for wall | Trump believes Obama would have started war with North Korea | Pentagon delivers aid for Venezuelan migrants Sarah Sanders says she was interviewed by Mueller's office Trump: I believe Obama would have gone to war with North Korea 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 HatchOrrin Hatch Foundation seeking million in taxpayer money to fund new center in his honor Mitch McConnell has shown the nation his version of power grab Overnight Health Care — Presented by PCMA — Utah Senate votes to scale back Medicaid expansion | Virginia abortion bill reignites debate | Grassley invites drug execs to testify | Conservative groups push back on e-cig crackdown MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerSasse’s jabs at Trump spark talk of primary challenger RNC votes to give Trump 'undivided support' ahead of 2020 Sen. Risch has unique chance to guide Trump on foreign policy 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 McConnellGreen New Deal Resolution invites big picture governing ‘Contingency’ spending in 3B budget deal comes under fire Coulter defends Paul Ryan: This is 100 percent Trump's fault MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerHouse Judiciary Dems seek answers over Trump's national emergency declaration Mandatory E-Verify: The other border wall Trump says he 'didn't need to' declare emergency but wanted 'faster' action 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 — Sponsored by America's 340B Hospitals — Dems blast rulemaking on family planning program | Facebook may remove anti-vaccine content | Medicare proposes coverage for new cancer treatment Overnight Health Care — Presented by PCMA — Senators seek answers on surprise medical bills | Red states move to limit Medicaid expansion | Two drug companies agree to testify Senate Dems block Sasse measure meant to respond to Virginia bill 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 MarkeyGreen New Deal Resolution invites big picture governing We can have a Green New Deal, and air travel too 2020 Dem slams Green New Deal: As realistic as Trump's claim that Mexico will pay for wall 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 GreenTrump taps Commerce watchdog to be new Interior inspector general Here are the lawmakers who will forfeit their salaries during the shutdown Christians at risk of extinction in land where Christmas began 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 TrumpTrump nominates ambassador to Turkey Trump heads to Mar-a-Lago after signing bill to avert shutdown CNN, MSNBC to air ad turned down by Fox over Nazi imagery MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnTrump’s new Syria timetable raises concern among key anti-ISIS allies Dem lawmaker invites Parkland survivor to attend State of the Union Bipartisan senators press Trump for strategy to protect Syrian Kurds MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.