Tennessee Sen. Bob Corker announced Tuesday he will not seek re-election in 2018. 

“When I ran for the Senate in 2006, I told people that I couldn’t imagine serving for more than two terms,” the Senate Foreign Relations chairman said in a statement.

“Understandably, as we have gained influence, that decision has become more difficult. But I have always been drawn to the citizen legislator model, and while I realize it is not for everyone, I believe with the kind of service I provide, it is the right one for me,” Corker said. 

The Tennessee Republican went from campaigning with President Donald Trump and wanting to be his secretary of State to saying the president lacked competence in the wake of his comments about the unrest in Charlottesville, Virginia.

Trump won Tennessee by 26 points last fall, and in response to Corker’s criticism, he tweeted that the state wasn’t happy with its junior senator. But in a meeting earlier this month, in which Corker asked Trump to campaign for Alabama Sen. Luther Strange, the president reportedly asked Corker to seek a third term.

The two-term senator already had one declared primary challenger, with several more taking a look at the race. 

But with an open seat, the primary field will likely grow. As a mostly single-party state, Tennessee has a history of crowded GOP primaries. In 2006, Corker defeated two former congressmen, Van Hilleary and Ed Bryant, to win the nomination for the open seat vacated by retiring Senate Majority Leader Bill Frist. Just last year, when a seat opened up in the 8th District, 13 Republicans competed. 

All eyes are now on 7th District Rep. Marsha Blackburn, whom some expect to announce a Senate bid this week. She had $3.1 million in the bank at the end of the second quarter. 

Blackburn was mum about her plans Tuesday afternoon.

“I don’t know. We’ll see,” she said when asked if she’ll run. 

Tennessee Republicans had regarded state Sen. Mark Green as the most serious primary threat to Corker. He was in Alabama on Monday night with former White House chief strategist Steve Bannon. But if Blackburn runs for Senate, Green could decide to run for her open 7th District seat.

Multiple GOP operatives in Tennessee have suggested that former 8th District Rep. Stephen Fincher isn’t done with politics and could also be interested in running for Corker’s seat. A strong advocate for the Export-Import Bank, he retired at the end of the last Congress after splitting with leadership over reauthorization of the bank. But he still has $2.3 million in his campaign account.

Corker said Tuesday that the remaining months of his term could well witness “the most important public service I have to offer our country.”

“I want to be able to do that as thoughtfully and independently as I did the first 10 years and nine months of my Senate career,” he said. 

Bridget Bowman contributed to this report.