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(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 is the Republicans' go-to sparring partner for mock presidential debates. Since 2000, he has played the roles of Al Gore, Joe Lieberman, John Edwards and Barack Obama.
Portman was careful not to discuss his current debate prep with Mitt Romney, since it is ongoing. Still, he said he has never agreed to an interview about his recurring Democratic role, until now.
Portman said he spends hours reading, studying and watching everything he can get his hands on to best embody Democrats and to prepare Republicans for debates.
Though he declined to portray his Obama imitation, those who have seen it say it is uncanny.
"He gets the style down, he gets the substance down, and he can really make you believe that he's Barack Obama," said Brett O'Donnell, a GOP debate coach who has worked with Portman over the years.
"He works very hard to understand every line of argument that the president might make and between those two characteristics, there isn't anyone better," O'Donnell said.
GOP sources say Portman's no-holds-barred style has been especially important during his countless hours with the former Massachusetts governor, since Romney sources admit one of his potential weak spots is becoming overly defensive, which can result in a gaffe.
A Romney adviser tells CNN his mock debates with Portman have gotten so intense that they make a point to build in breaks for him to decompress with his family.
Inside Romney's mock debates, which began a month ago at a friend's home in rural Vermont, aides say they do their best to create what spokesman Kevin Madden calls "game-day conditions."
That means practicing the best way to talk policy, but also making sure the room is set to feel as close to the real thing as possible: two podiums and a moderator, played by longtime Romney adviser Peter Flaherty. Members of Romney's inner circle, including Beth Myers, Stuart Stevens and Ed Gillespie, then dissect the answers and look for areas of improvement.
O'Donnell says it's impossible to underestimate the importance of the psychological aspect of debate prep.
"There's three things that you have to do to prepare a candidates: they have to know the policy material, they have to know the answers to the questions. The second thing is they have to know the strategy and how to execute it. But the third thing and probably the most important is they have to be mentally prepared.
"It's just like preparing an athlete, if they're not mentally prepared for the worst of the worst then something will take them by surprise and so those mock debates are extremely important because I look at it like a batter in the on deck circle, they swing a heavy bat or a bat with a weight on it so that when they get in the batter's box the bat feels lighter," O'Donnell explained.
Part of the prep is not just what Romney says; it's how he acts and reacts.
Portman remembers preparing to play Al Gore in mock debates with George W. Bush during the 2000 campaign, and noticing that Gore got unusually physically close to his opponent, Bill Bradley.
So Portman, acting as Gore, invaded Bush's personal space during rehearsal.
"Governor Bush's reaction was, 'He is not going to do that, that is ridiculous,' and sure enough in that last debate, which was the one we were prepping for, Al Gore did just that. I saw Governor Bush smile. I wonder if he was smiling saying 'Darn, Portman was right' or if he was just smiling at Al Gore, but he handled it well," Portman recalled.
O'Donnell, who was brought into the Romney campaign after a few underwhelming debate performances during the GOP primaries, says the candidate is extremely responsive to critiques.
"He is a great student of political debate. He works very hard at it, so I think that he is absolutely coachable. He was able to do things across the debates, change a strategy, that let him win a majority of these debates," O'Donnell said.
O'Donnell considers the CNN debate in Jacksonville, Florida, one of Romney's great debate moments -- when Newt Gingrich refused to criticize Romney about lacking transparency as the former speaker had so often done on the campaign trail.
Romney jumped in and asked, "Wouldn't it be nice if people didn't make accusations somewhere else that they aren't willing to defend here?"
"It took Gingrich's argument and showed the absurdity in one line," O'Donnell said. "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 Romney has to be looking for now," O'Donnell said.
GOP sources say Romney has been working up a few one liners to get his points across, but O'Donnell also said he hoped the Romney campaign isn't also practicing "zingers" as The New York Times reported over the weekend.
"It's not about zingers," O'Donnell said.
"A zinger might be just something funny, a comedic line, something to sort of break the monotony of the debate to try and capture the imagination," he said. "It's something that's sort of shoehorned into the debate.
"But a good line, Reagan's line about his opponent's youth and age, not making that an issue, a good line is a substantive line that does capture the imagination of the press and the audience, to overrun and deliver the message that you want and do it at the expense of your opponent."