Sen. Rob PortmanRob PortmanRepeal of Obama drilling rule stalls in the Senate Chamber pressures red-state Dems to back Trump on cutting regs GOP govs: ObamaCare repeal bill shifts 'significant' costs to states MORE (R) says he is not running for president in 2016, ending speculation around the centrist Ohio senator. 

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"I don't think I can run for president and be an effective senator at the same time," he said in a statement late Monday night, announcing that he will be seeking reelection to the Senate in 2016.

Portman was mentioned as a potential GOP vice presidential pick in 2012 and comes from the crucial swing state of Ohio. He said that he hoped to make progress in the Senate on issues like tax reform, increasing exports and energy production.

Portman also served as director of the Office of Management and Budget under President George W. Bush. 

Portman supports gay marriage, a stance that could have been a point of contention in a Republican presidential primary. 

"It really wasn't a factor," he told The Associated Press Monday night. "Some people say it would have hurt me. Some people say it would have helped me. The country is obviously moving on that issue.

"Truth is, most people are much more focused on other issues, including jobs and the economy," Portman said.

As for staying in the Senate, he said, "I'm excited about it."