Ohio Republican Sen. Rob Portman won't run for President in 2016, he announced late Monday night in a statement.

Portman said he is instead planning for his 2016 reelection to the Senate, where he thinks he can "play the most constructive role."

"I don't think I can run for president and be an effective senator at the same time," Portman said. "While I appreciate the encouragement I have received from many to run for president, my focus will remain on Ohio and running for re-election to the Senate in 2016."

Portman has been viewed as a potential contender for president in 2016 ever since he was vetted, but ultimately not picked, to be Mitt Romney's running made in 2012.

Portman would have been the first pro-gay marriage Republican presidential candidate. The Ohio Senator announced his support for same-sex marriage in 2013, two years after his son came out as gay.

He said as recently as two weeks ago that he was considering a presidential run, telling CNN last month that "I probably have more experience than other people who are running or thinking about running."

But Portman made it clear in his statement that he believes he can be a more effective leader while in the Senate as Republicans preside over both chambers starting in 2015.

"With the new Republican majority, I see a real opportunity over the next two years to break the gridlock in Washington and actually get things done to help Ohioans and all Americans," Portman said. "I look forward to making progress on stalled policies to jump-start our economy, increase family paychecks, and restore the American Dream."

Portman will formally announce his Senate reelection bid next year.