I'm not sure what it would take to convince Palin and McCain that insinuating another candidate for the presidency might somehow be a secret terrorist is a very, very bad idea. America has had plenty of experience with right-wing militia nuts and paranoid loners over the last few decades; it should be transparently obvious that sneeringly inciting crowds to such an extent is a dangerous game for Palin and McCain to be playing. Palin, especially, should be damn well aware of it, given her and her husband's past uncomfortable closeness to militia figures in Alaska.
To put it plainly, inciting right wing crowds to consider an American presidential candidate a "terrorist" isn't merely dirty politics or otherwise unseemly -- it comes uncomfortably close to implicitly encouraging actual violence. And that "uncomfortably close" part is putting the most charitable possible face on it.
I'm sure there's no way we can make them stop, or even tone down their at this point intensely personalized, intensely spiteful rhetoric. They are immune to shame, or even common sense; this is the last desperate hour of the McCain campaign, and Palin was selected specifically because she could make these sorts of sleazy, least-common-denominator appeals to a far-right base that needs it, in order to feel good about voting for McCain.
But I think an adequate karmic retribution for Palin would be, if and when Obama is elected, for her to be made his personal assistant and be ankle-chained to him every time he leaves the White House -- so that whatever happens to Obama during his presidency happens to her as well. Maybe then she'd gain a shred of introspection into how much you should or shouldn't actively incite dumbass, already paranoid, explicitly racist crowds into thinking that an American president is an enemy of the state.