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.