Democratic primaries are actually pretty dynamic:
Carter/Brown
Carter/Kennedy
Mondale/Hart
Dukakis/Jackson
Clinton/Brown
Clinton
Gore/Bradley
Kerry/Edwards
Obama/Clinton
Obama
Hmm…
Ford/Reagan
Reagan/Bush
Reagan
Bush/Dole
Bush/Buchanan
Dole/Buchanan
Bush/McCain
Bush
McCain/Romney
Romney/Santorum