This is not an easy answer.
The modern GOP was born from the ashes of the Barry Goldwater campaign. He was a Republican's Republican - small government, states' rights, fuck the blacks, unions and poor. he was also a principled man of admirable character who got absolutely annihilated by LBJ.
This caused some soul-searching amongst the leading lights of the Republican Party. Their result: they had no soul. When it became abundantly clear that they could not win a majority for what they stood for, they opted to focus on what they stood against in order to spook the shit out of white people.
It's interesting that you mention Nixon and Dubya but not Reagan or George H. Their campaigns were run by Lee Atwater who died remorseful of a brain tumor (he was a sack of shit). George W's campaign was run by Karl Rove, who would go on to run the College Republicans after working on the Nixon campaign.
Of course, none of the above talked about "death panels" as Palin did during McCain's election. And nobody suggested that their opponents were criminals (while being investigated by the FBI). So it's kind of a matter of degree, isn't it?
Nobody really called Romney out as a dire scumbag. He was a Republican but not a terrible one. And nobody had much awful to say about Bob Dole, either.
So on the one hand, you could argue that the Republicans burned to the ground in '64. But on the other hand, you could argue that the only time they win is when they've got a goddamn conflagration.
...unless the opponent is Dukakis.