I don’t forget about Roosevelt, who served for 12 years alone. But he had a Depression, which the people (horribly wrongly) interpreted as justifying a benign King to rule over us. Nothing like the Depression will sweep Republicans into office fro twenty years, because when big, bad things happen people look for someone to take care of them, and that runs counter to our philosophy. No Depression, no FDR, just like no Civil War, no Republican frenzy across the turn of the centruy.
As for the counter-historical uninterrupted Republican run starting with Nixon, I’ll admit, Republicans have done especially well in the presidency since the liberals went crazy in the sixties. However, Carter was almost inevitable given how badly Nixon was portrayed in the media. And Reagan, who took it back from the sixties-favorite Carter was different enough from Nixon to almost be from another party. And Dukakis was simply a pathetic candidate.
Granted, Nixon and Reagn were enormously popular in their time, and Carter was unpopular. But the failure of Republicans to continue in the Reagan tradition after Bush I was elected goes toward proving my point, I think. No single party can, outside of special circumstances, hold onto power for an entire generation.