Republican presidential candidate and former Massachusetts Gov. Mitt Romney talks with Sen. Rob Portman, R-Ohio, who has also been his debate practice partner, on his campaign charter plane en route Denver, Monday, Oct. 1, 2012. (AP Photo/Charles Dharapak)

Photo by: Charles Dharapak

Republican presidential candidate and former Massachusetts Gov. Mitt Romney talks with Sen. Rob Portman, R-Ohio, who has also been his debate practice partner, on his campaign charter plane en route Denver, Monday, Oct. 1, 2012. (AP Photo/Charles Dharapak)