The Tea Party Republicans are a counter-example (or more accurately, a counterinsurgency). While there are still only two parties, one of them has become something that would be nearly unrecognizable to its members from the '70s.

It is possible to organize within the party to bend it. But in general, one side of the aisle tends to seem to have difficulty with finding enough common ground to actually work as a bloc, while the other side has managed, impressively, to unify Christian fundamentalists and ultra-rich billionaires.