MITT Romney's eldest son Tagg has told a radio station that the back-and-forth debate sniping between his father and Barack Obama made him want to "take a swing" at the president.

Asked in an interview with North Carolina radio host Bill LuMaye what it was like to sit through Tuesday's super-charged debate and hear Obama "call your dad a liar", Tagg Romney said it made him want to "jump out of your seat and you want to rush down to the debate stage and take a swing at him".

He continued with his description of how the extraordinarily close race is weighing on those close to the protagonists, less than three weeks before Americans head to the polls on November 6.

He followed up his joke about clocking Obama by saying: "But you know you can't do that because, well first because there's a lot of Secret Service between you and him, but also because it's just the nature of the process.

"They're going to try to do everything they can do to try to make my dad into someone he's not," Tagg, the eldest of Romney's five sons, said.

"We signed up for it. We gotta kind of sit there and take our punches, and then send them right back the other way."

There was no malicious tone to his voice, and a campaign aide said that Romney was clearly speaking in jest.

"He was joking about how frustrating this process can be for family," the aide told AFP.

The political slugfest has heated up in recent weeks, and it reached a peak at Tuesday's presidential debate at a New York university, where Obama and Romney often stood just feet apart, talking over each other.

Asked if his father gets nervous before such debates, the younger Romney said: "Absolutely, are you kidding? He's terrified before he gets out there.

"Terrified is too strong a word, but you know, like anybody, he gets butterflies a little bit," Romney added.

Tagg's younger brother Josh made light of the incident when he appeared with his mother Ann Romney on TV talk show The View.

Tagg "has slugged me a couple times. I assure you, President Obama has nothing to worry about," he quipped.

"We're just fighting hard for my dad, we want to get him elected," Josh added.

He, too, spoke of the personal difficulties of watching a loved one endure a heated presidential campaign.

"To watch him get up there and go through a tough debate ... you really don't like to see your dad get beat up by the media or President Obama or whatever it is, and so you take it pretty personally," he said.

"But I think that was just something he was saying off the cuff, and I assure you he didn't mean it."