"Then the minority-turned-majority party stops trying so hard to please the voters, and begins drinking the wine of power. That's my best guess for why the balance of voters tends to be actively restored around a 50/50 balance."
I think the disadvantage of the majority party is, that they are able to make mistakes. And this, rather that being corrupted by the power, drives the balance toward 50/50.
Perhaps you should write someday about choosing examples. In certain circumstances the computing power wasted is a good measure of intelligence, for example if one considers chess programms.