Tagg Romney was speaking to the Bill LuMaye Show, a talk radio show in North Carolina, about his father's appearance to the nation on Tuesday night (October 16).
When asked how he felt about Obama calling his father a "liar", Tagg Romney replied: "[You] jump out of your seat and you want to rush down to the debate stage and take a swing at him, but you know you can't do that because there's a lot of secret service people between you and him.
"But also because this is the nature of the process. They're going to try to do everything they can to make my dad into someone he's not. We signed up for it. We've got to kind of sit there and take our punches, and then send 'em right back the other way."
Tuesday's second live debate saw Obama stage a comeback after Mitt had appeared to win the first on October 3. Mitt also caused controversy by making a "binders full of women" comment when replying to an undecided voter's query about equal pay.
Tagg added that his father gets "butterflies" before debates, but praised the Republican nominee's efforts.
"I think he did a very good job of letting people know that not only can he talk the talk, but he can walk the walk," Tagg said. "Once he's in it, two or three minutes in, he's forgotten about the nervousness and he's amped up and ready to go."
He also said that he and his siblings Matthew, Joshua, Benjamin and Craig "have all made a pact not to give him advice", adding: "Our role is to go out and speak on his behalf."