Michael BÃ©rubÃ© explains it all in a 12-step cycle we are doomed to repeat.
1. Democratic president is elected after disastrous Republican administration messes things up bad. Liberals rejoice, hoping that their long national nightmare is now over.
2. Democratic president turns out to be liberal–centrist fellow with some degree of cultural conservatism and willingness to echo Republican talking points on a handful of issues.
3. Democratic president meets with solid Republican opposition in Congress as well as various forms of obstructionism from members of his own party.
4. Democratic president gives in to Republicans repeatedly on a handful of symbolic (and therefore important to politically active voters) issues, appointments, regulations, etc.
5. Left wing of Democratic party erupts in outrage at sellout Judas stealth-Republican president.
6. Portion of left wing of Democratic party leaves party, goes home, fantasizes about awesome third party that will destroy the system and rebuild it from scratch.
You will have to go here for 7 through 12.