Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) has decided not to run for reelection in 2018 after reconsidering his decision to retire.

Corker, the chairman of the Senate Foreign Relations Committee, announced his decision to retire last September. But in recent months, Republicans in Tennessee and in the Senate had urged him to consider running for a third term, according to his chief of staff.

“Based on the outpouring of support, we spent the last few days doing our due diligence and a clear path for re-election was laid out," said chief of staff Todd Womack.

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"However, at the end of the day, the senator believes he made the right decision in September and will be leaving the Senate when his term expires at the end of 2018."

A reversal from Corker would have created a high-profile primary clash with Rep. Marsha Blackburn Marsha BlackburnDemocrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections GOP senators unveil new bill to update tech liability protections MORE (R-Tenn.), who announced her bid for the seat after Corker's retirement. Some Republicans had expressed concern that Blackburn, a conservative lawmaker, would have difficultly in a general election match-up against former Tennessee Gov. Phil Bredesen, the likely Democratic nominee.

But Blackburn made clear she planned to remain in the race regardless of Corker’s decision. Her campaign slammed those who doubted her ability to win in November as “plain sexist pig[s].”

As speculation swirled that Corker might run again, Blackburn’s campaign rolled out a series of endorsements and groups supporting her and touted polls that showed her defeating Corker in a primary.

Corker had been an ally of President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE during the 2016 campaign, but their relationship became strained last year after Corker compared the White House to "an adult day care center."

Trump tweeted that the senator “begged” him for an endorsement and “dropped out” when he refused to give it.

Since then, Trump and Corker have appeared to smooth things out. Still, the White House indicated to Corker's team that Trump wouldn’t make an endorsement in a primary match-up between the senator and Blackburn, according to Politico.

Corker's official exit leaves Blackburn in a strong position to avoid a difficult primary on her way to the Republican nomination. She’s been a strong fundraiser during her Senate bid, bringing in more than $2 million in the last three months of 2017.

In a statement following the news, Blackburn thanked Corker for his time in the Senate and said that Tennessee Republicans can now shift their focus to the general election.

"Now, we can unify the Republican Party and focus on defeating Democrat Phil Bredesen in November,” Blackburn said in a statement. “As we continue to take our campaign to every corner of the state, I'm looking forward to listening to Tennesseans families and sharing my ideas on how we can get the United States Senate back to work and pass President Trump’s agenda."

Updated at 11:58 a.m.