(CNN) -- Republican Sen. Rob Portman gets so into character playing Barack Obama during debate prep that his performance four years ago sparring with then-GOP presidential candidate John McCain made his wife, Cindy, run out of the room crying. "Part of your responsibility in these debates is to be tougher so the candidate you are helping is ready for the worst of it -- so you have to be mean. You need to get under their skin. Sometimes the candidate you are working with doesn't appreciate it, and even more often, their family doesn't appreciate it," Portman told CNN in an exclusive interview.Portman has portrayed Obama in Romney's debate preparationsOhio senator has acted as Democratic candidate in mock debates since 2000"He can really make you believe he's Barack Obama," GOP debate coach saysRomney adviser says mock debates are so intense, they had to build in time to decompress
Though he declined to portray his _ imitation, those who have seen it say it is uncanny. "It created a moment in the debate where through one line, he could capture the imagination of the press, capture the imagination of the audience, and make a point -- make a strong point that crystallized that entire debate in one line, and that's what Governor _ has to be looking for now," O'Donnell said.