Portman -- who stands in as President Barack Obama during Romney's debate prep -- said in an interview on ABC's "This Week" he believes the momentum is on Romney's side in Ohio.
"If you look at the average of all the polls, it's about dead-even in Ohio right now," Portman said.
"I've been to a half-dozen rallies in Ohio in the last week alone. I've never seen this kind of energy and enthusiasm on the ground. ... And I think that's why you're going to see the president continue to attack, not focus on the substance of the issues that people care about, but instead continue to get attacked because things are not going their way right now," he added.
Portman said he believes Obama "is going to come out swinging" at Tuesday night's presidential debate "to compensate for a first debate." He said he anticipates Obama will continue to run a "highly negative ad campaign."
"They've spent hundreds of millions of dollars around the country, including a lot in Ohio, mischaracterizing Governor Romney's positions and misrepresenting him," Portman said. "And I think you'll see that again at the debate on Tuesday night."