Former president Bill Clinton swatted down Republican suggestions that wife Hillary's health could hamper her potential White House run, insisting Wednesday she is in "better shape" than him.

"She works out every week, she is strong, she's doing great," Clinton said at a fiscal forum in Washington.

"As far as I can tell she is in better shape than I am. She certainly seems to have more stamina now."

Should she choose to run, the 66-year-old former secretary of state would be the early Democratic frontrunner in the 2016 presidential race, and Republicans have made no secret of organizing opposition to her candidacy.

But her husband scoffed at Republican strategist Karl Rove's suggestion that Clinton suffered a "traumatic brain injury" during a 2012 fall that caused a concussion.

"First they said she faked her concussion," Bill Clinton said, alluding to Republican accusations she had exaggerated her injury to delay testifying about a terrorist attack on the US mission in Benghazi, Libya that killed four Americans.

"Now they say she's auditioning for a part on The Walking Dead. I mean, you know, whatever it takes," Clinton added with a grin.

While he said "there's nothing" to the suggestions his wife has been mentally incapacitated, he turned serious in recognizing the severity of the injury.

It "was a terrible concussion that required six months of very serious work to get over," he said, adding that it was something she "never tried to pretend didn't happen."

"Now they say she's really got brain damage," Clinton said, returning to humorous mode. "If she does then I must be in really tough shape because she is still quicker than I am."

Clinton chalked up Rove's remark to the intensity of American politics and Republicans' efforts to publicize doubts about their main rival's qualifications.

"It raises a serious issue, even in a ridiculous way," Clinton said.

"You can't be too upset about it. It's just the beginning; they'll get better and better at it."