WASHINGTON (AP) — Republican Sen. Bob Corker said Monday he is unsure about running for re-election, injecting fresh doubts into the GOP's efforts to tighten its hold on the Senate majority next year.

The Tennessee lawmaker, a sometimes critic of President Donald Trump, expressed his ambivalence in a brief statement, even as he sits on a hefty campaign account for next year's race. Corker has $7.5 million cash on hand, according to Todd Womack, a senior aide to the senator.

"While we are in a strong position, I am still contemplating the future and will make a decision at the appropriate time," Corker said.

He emphasized that "everyone in the Volunteer state knows ... running for re-election has never been an automatic for me."

Corker, 65, chairs the influential Foreign Relations Committee and has been regarded as a shoo-in for a third term. His name surfaced as possible secretary of state in the Trump administration, a job that went to Rex Tillerson.

Corker 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."

Republicans hold a 52-48 edge in the Senate and are playing offense next year, with 23 Democrats and two independents up for re-election compared with nine Republicans. Ten Democrats represent states Trump won in 2016.

But Corker's uncertainty comes as incumbent Republicans in Arizona and Nevada face primary challenges, and Alabama Sen. Luther Strange is locked in a tough run-off against Roy Moore. That election is Sept. 26.