Sen. Bob Corker says he's not sure if he'll run for re-election.
The Tennessee Republican, a sometimes critic of President Donald Trump, said in a statement Monday that "running for re-election has never been automatic for me."
Corker, 65, chairs the influential Foreign Relations Committee and has been regarded as a shoo-in for a third term.
He raised eyebrows last month when he told Tennessee reporters after a town hall meeting that Trump "has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful."
Trump responded with a swipe on Twitter calling it a "strange statement."
Corker has more than $7.5 million in his campaign account but could face a GOP primary challenge.