Sen. Portman discusses role-playing Obama in GOP debate prep

He has also played former Sen. John Edwards (N.C.), former Vice President Al Gore and Sen. Joe Lieberman (I-Conn.) in debate prep for previous elections. He said "the key is to be tougher than they are" when playing the role of an opponent.

He promised to play an aggressive Obama opposite Romney in his practice sessions.

"I think Mitt Romney's got a great message, so President Obama's going to have to attack," he predicted.

Portman's name most recently came up in speculation regarding Romney's running mate before Rep. Paul Ryan (R-Wis.) was picked.

"It was a shortlist, but apparently not short enough," he joked when asked about the VP talk.

The Obama campaign is using Sen. John Kerry (D-Mass.) to play Romney in Obama's debate prep and ranking House Budget Committee member Chris Van Hollen (D-Md.) in Vice President Biden's debate practice for when he goes up against Ryan.