Former Tennessee Gov. Bill Haslam has decided he won’t seek to replace retiring United States Sen. Lamar Alexander in the 2020 election.

An op-ed written by Haslam appeared on the Tennessean’s website Thursday announcing the decision not run.

Both Haslam and Alexander are Republicans, and each served two terms as the Tennessee state government’s highest elected officeholder.

“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 said. “This is a difficult decision because I have loved my time in public service and I believe so deeply in the importance of our political process.”

Haslam said wrestling “with the possibility of running for the United States Senate” has been “the hardest vocational decision of my life.”

As yet, only Vanderbilt surgeon Manni Sethi has announced he’s running for the Republican nomination to replace Alexander. Others said to be considering joining the race include U.S. Ambassador to Japan Bill Hagerty, freshman GOP U.S. Rep. Mark Green*, former Congresswoman Diane Black and U.S. Rep David Kustuff.

James Mackler, a Nashville attorney, is running as a Democrat.

*Update: Green announced later Thursday that he won’t run for Alexander’s Senate seat.