Jimmy Carter put himself in harm's way while in the military to close down a runaway nuclear reactor. He did the same as President, symbolically, during the Three Mile Island Crisis to calm the nation's fears.
Carter also ordered an ill fated rescue of US Hostages held in Iran. The rescue failed only because the equipment used failed.
Apparently, Mitt Romney is unaware that neither Carter's brains nor his bravery is subject to question. The same can not be said of Romney as he repeatedly demonstrates.