Sen. Lamar Alexander (R-Tenn.) said he decided that his three Senate terms and two as governor were enough. “Everything has to come to an end,” he said. (Mark Humphrey/AP)

Sen. Lamar Alexander (R-Tenn.) announced Monday that he would not seek reelection in 2020, a move likely to bring a close to a long political career characterized by his ability to seek consensus and work with politicians from both parties.

His departure from the Senate will further deplete the ranks of Republicans inclined to cooperate with Democrats. Congress has grown increasingly partisan in recent years, and during President Trump’s tenure, some Republicans who don’t share Trump’s combative approach have headed for the exits.

Alexander, 78, a former Tennessee governor and U.S. education secretary who twice ran for president, said in an interview that he made up his mind in August while he was fishing in Canada.

The senator said he concluded that his three Senate terms and two as governor were enough. “Everything has to come to an end,” he said, adding that it is time for somebody else to step in.

Alexander has cultivated a reputation for being a traditional Republican senator, voting with Trump much of the time but willing to work with Democrats.

Alexander said he waited until the end of the year before announcing his decision to ensure it was the right one. He called Trump on Sunday to inform him, and before he could reveal his decision to the president, Trump talked about him serving 20 more years, Alexander recalled.

When he told Trump why he was calling, the president expressed disappointment, according to Alexander, but also congratulated him.

A close ally of Senate Majority Leader Mitch McConnell (R-Ky.), Alexander serves as chairman of the Senate Health, Education, Labor and Pensions Committee. He was at the center of the unsuccessful 2017 push to repeal and replace the Affordable Care Act, known as Obamacare.

Alexander’s decision means that for the second time in two years, Tennessee will have an open U.S. Senate race.

Sen. Bob Corker (R-Tenn.), a Trump critic, announced last year that he would retire at the end of this year. Corker’s successor will be Rep. Marsha Blackburn (R), a conservative and staunch ally of Trump.

In a statement Monday, Corker called Alexander “one of the finest statesmen our state has ever seen.”

Alexander drew Republican primary challengers in his last election, including Joe Carr, a tea party insurgent who ran to his right. Alexander won that contest comfortably.

A poll released this month found that Alexander remains popular in his home state. Sixty-five percent of Republican primary voters in Tennessee had a favorable view of him, while 22 percent had a negative view, according to the poll by North Star Opinion Research.

In Monday’s interview, Alexander said Washington had turned into a “split screen,” with fierce partisan fights like the battle over Supreme Court Justice Brett M. Kavanaugh’s confirmation alongside bipartisan endeavors, such as a bill to fight the opioid epidemic.

“I think there’s plenty that goes on in a bipartisan way, and I think the more senators we have who use those skills and work with each other, the better the Senate will be,” he said. He added that bipartisanship is harder in today’s environment.

Nationally, Republicans face a more difficult Senate map in 2020 than they did this year, when Democrats had to defend 10 seats in states Trump won. Republicans will be defending 22 seats in 2020, compared with 12 for Democrats. Alexander’s decision means Republicans will have to find a new candidate in a state where Trump won by a wide margin in 2016 and remains popular.

Republicans are already thinking about potential candidates for the open seat. Alexander pointed to Gov. Bill Haslam, who is about to leave office, and Bill Hagerty, the ambassador to Japan. Rep.-elect Mark Green and Rep. David Kustoff are also possibilities.

“We’ve got a barn full of talent in Tennessee,” Alexander said.

In this year’s Senate race, Democrats backed former governor Phil Bredesen, a moderate who was seen as their best chance of winning statewide in years. But after a promising start to his campaign, Bredesen proved to be no match for Blackburn, who aligned herself closely with Trump.

In a statement Monday, Blackburn called Alexander “an effective advocate for Tennesseans” and said his “leadership is a model to be emulated.”

After serving two terms as governor and three years as president of the University of Tennessee, Alexander was U.S. education secretary from 1991 to 1993 under President George H.W. Bush.

Alexander unsuccessfully sought the presidency in 1996 and again in 2000. He won election to the Senate in 2002.

He was the principal author of the 2015 Every Student Succeeds Act, which was signed into law by President Barack Obama, replacing the No Child Left Behind Act.

During a floor speech last year after Republican efforts to repeal the Affordable Care Act failed, Alexander notably told colleagues that it was time to change course and work with Democrats to reform the law.

He warned that letting Obama’s signature law collapse under its own weight, as Trump and many conservatives had advocated, would be a mistake.

“I would ask what’s conservative about unaffordable premiums?” he said. “What’s conservative about creating chaos so millions can’t buy health insurance?”

Seung Min Kim contributed to this report.