NASHVILLE (AP) — Former Republican Tennessee Gov. Bill Haslam won't run for the U.S. Senate seat being vacated by GOP Sen. Lamar Alexander, a move sure to draw new interest in the race from other would-be candidates.

When Alexander announced in December that he wouldn't seek a fourth term in 2020, Haslam said he would give "serious consideration" to making a Senate bid.

Six months later, Haslam finally declared his intentions, jokingly apologizing "to all those who think it has taken me an exceedingly long time to make up my mind!"

"The truth is, this has been the hardest vocational decision of my life," he explained in a letter he first sent to The Tennessean.

"While I think serving in the United States Senate would be a great privilege and responsibility, I have come to the conclusion that it is not my calling for the next period of my life," Haslam wrote.

The announcement did not spur an immediate campaign launch from a prominent Republican. Instead, GOP U.S. Rep. Mark Green told supporters in an email after Haslam's decision Thursday that he won't be running for the Senate seat, either.

This marks the second time in two years that Haslam backed away from a Senate campaign. He also declined to run to replace retiring GOP Sen. Bob Corker, whose seat was won in November by Republican Sen. Marsha Blackburn. Haslam said then that he didn't want the distraction during his remaining time as governor.

Alexander, for one, said he understands how Haslam feels.

"Bill Haslam would be a terrific United States senator, but he has served Tennessee well and unselfishly for 15 years as governor and as mayor of Knoxville, and I can completely understand his desire to take a different direction with his life," Alexander said in a statement Thursday.

Haslam's decision leaves the Senate field in flux for the Republican state. The billionaire had shown a willingness to self-fund campaigns and he left the governor's office with high popularity.

Trauma surgeon Manny Sethi announced his entrance into the race last month. Sethi's campaign said Thursday that he entered July with more than $1.5 million cash on hand after loaning his campaign $1 million and raising another $550,000.

Sethi thanked Haslam for his service in a tweet Thursday and added that Haslam's decision "taught us something today: the Founders never wanted to have a permanent political class. They wanted patriots who would serve for a time, then return to private life."

Former U.S. Rep. Diane Black told the AP on Thursday that she isn't interested. Black lost in a crowded Republican primary for governor last year to new GOP Gov. Bill Lee.

Green is opting against a run, despite early support from Club for Growth, a conservative group that had already begun waging preemptive attacks against Haslam.

U.S. Ambassador to Japan Bill Hagerty's name is being floated for the seat, even by Alexander himself. Hagerty was once Haslam's economic development commissioner and would have been unlikely to run against his old boss.

U.S. Rep. David Kustoff tweeted Thursday that he's been approached by people in Tennessee encouraging him to run, saying he looks forward to "continuing to talk to the people about how to best continue serving our great state."

Other Republicans also could join the mix.

On the Democratic side, Nashville attorney and former Army helicopter pilot James Mackler announced his candidacy in January. Mackler dropped out of the race for Corker's seat when former Democratic Gov. Phil Bredesen opted to run, only to see Bredesen lose by about 11 points to Blackburn. It was a disheartening blow to Democrats who thought they had their best shot at a statewide elected office in years.

Through March, Mackler raised more than $270,800 and spent $187,700, leaving him with about $92,300 cash on hand. His campaign has not released campaign finance totals through June, which are officially due on Monday.

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Kimberlee Kruesi in Nashville contributed to this report.