WASHINGTON ― Sen. Lamar Alexander (R-Tenn.) made a surprise announcement Monday that he will not be seeking re-election 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. I am deeply grateful, but now it is time for someone else to have that privilege,” Alexander, who was twice elected governor of Tennessee, said in a statement.

Alexander, 78, was first elected to the Senate in 2002. He currently chairs the powerful Health, Education, Labor, and Pensions Committee and is a close ally of Senate Majority Leader Mitch McConnell (R-Ky.).

Most recently, Alexander authored legislation President Donald Trump signed into law aiming to confront the growing opioid epidemic in the country. Last year, amid a push to repeal the Affordable Care Act, Alexander teamed up with Sen. Patty Murray (D-Wash.) in a failed legislative attempt to stabilize the law’s health insurance exchange markets.

Bob Corker, another establishment Republican senator from Tennessee who is stepping down this year, hailed Alexander as the “legislator of the decade” and as “the finest statesman our state has ever seen.”

The senator’s announcement is disappointing news for the GOP, which is already facing a challenging electoral map in 2020. A recent poll found Alexander quite popular among GOP primary voters in Tennessee, with 69 percent approval.

Democrats still face an uphill climb in flipping the seat. The last Democratic Senate nominee in the state, Phil Bredesen, turned in a less-than-impressive performance, losing to Rep. Marsha Blackburn (R-Tenn.) by 11 points in the 2018 election.