- Power corrupts.
- More power = more corruption.
- Constantly voting the same party into office = more power for that party = more corruption.
Cases in point: since Nixon, there have been more Republican than Democratic presidencies. Not coincidently, the worst political scandals (Watergate, Iran-Contra and arguably the Iraq War) have involved Republican presidencies (Clinton lying under oath about head was bad, but it's hardly on the same level; Repubs needed to make it seem so cuz they're still smarting from Watergate).
The beauty of having more than one political party is that it provides some measure of checks and balances. When the party in power starts screwing up, you vote the other party in. When that party starts screwing up, you vote another party in (or in our case, you vote the first party back in).
Our government works best when voters think outside their little partisan box. Blind allegiance = fertile ground for political corruption.