Al Behrman/AP

In ruling out a presidential bid late Monday night, Rob Portman offered a bit of a warning message for several of his Senate colleagues who are considering White House runs of their own in 2016.

"I don't think I can run for president and be an effective senator at the same time," the Ohio Republican said in announcing that he will instead seek a second term in the Senate next year. Portman, 58, was on the short list to be Mitt Romney's running mate in 2012 and had been one of more than a dozen Republicans informally "considering" a presidential campaign in 2016. A former U.S. trade representative and budget director under President George W. Bush, he is well-respected on policy matters in the Senate and is seen as much less of a firebrand than a number of the more conservative members with ambitions for national office.

Portman's candidacy would have posed a test of whether a contender publicly supportive of same-sex marriage would stand a chance with conservative primary voters. But it was his rationale for passing on a presidential bid that has interesting implications on Capitol Hill. Senators Rand Paul, Ted Cruz, and Marco Rubio are all likely to enter the 2016 race next year, creating a dynamic that could make life difficult for GOP leaders trying to advance the party's agenda in the new Republican majority.

“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 in his statement. "That's where I believe I can play the most constructive role."

Paul announced Tuesday he will seek a second Senate term while preparing to mount a simultaneous run for the presidency beginning next year, a decision that will require him to challenge or otherwise work around a Kentucky law forbidding candidates from appearing on the same ballot for two different offices. Cruz's Senate term goes until 2018, while Rubio has already said he won't seek a second term in 2016 if he runs for president.

Portman has chosen to simplify things for himself, at least for the time being. He won't be on the top of the GOP ticket in 2016, but as a swing-state senator with a long résumé, he could once again find himself at the center of the vice-presidential sweepstakes.

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