It's not a case of having the cajones to do it, it's about having the means to do it. Look at the effort and planning that the Allies had to put in to invade France - and that was an invasion launched from Britain.
Germany would first have to gain control of the North Atlantic, which would mean defeating Britain and rendering the US Navy ineffective, and then acquiring a base from which to launch to launch the invasion, such as Cuba.
*And* this would have really have to have happened in 1940 or 1941, before the US's massive economic output could simply overwhelm German materiel resources.