Portman, who served as the director of the Office of Management and Budget during the George W. Bush administration, is one of Romney's top campaign surrogates, and was considered to be a frontrunner in the so-called "veepstakes" before Romney announced that Rep. Paul Ryan (R-Wis.) would be his running mate. CNN reported Monday that the campaign approached Portman about the debate prep role earlier this summer, and has been given preparation material.
In June, the Obama campaign announced that Sen. John Kerry (D-Mass.) will play Romney in the president's debate prep sessions.
The first presidential debate will take place on October 3 in Denver, Colo., and will be hosted by PBS' Jim Lehrer.