Sen. Bob Corker, R-Tenn., announced Tuesday he will not seek re-election to his seat in 2018, making him the first Republican senator to announce his retirement from the Senate ahead of the midterm elections.

A two-term senator and the former mayor of Chattanooga, Corker made the announcement in a statement Tuesday afternoon. He said that while he never imagined he’d stick around for more than 12 years in the Senate, the decision became more difficult due to the considerable influence he has accrued, particularly as chairman of the powerful Senate Foreign Relations Committee.