I've always thought that corruption is unavoidable in ruling parties - because corruption always starts with the best of intentions. David Brooks explains:
Back in 1995, when Republicans took over Congress, a new cadre of daring and original thinkers arose. These bold innovators had a key insight: that you no longer had to choose between being an activist and a lobbyist. You could be both. You could harness the power of K Street to promote the goals of Goldwater, Reagan and Gingrich. And best of all, you could get rich while doing it!
Before long, ringleader Grover Norquist and his buddies were signing lobbying deals with the Seychelles and the Northern Mariana Islands and talking up their interests at weekly conservative strategy sessions - what could be more vital to the future of freedom than the commercial interests of these two fine locales?
Before long, folks like Norquist and Abramoff were talking up the virtues of international sons of liberty like Angola's Jonas Savimbi and Congo's dictator Mobutu Sese Seko - all while receiving compensation from these upstanding gentlemen, according to The Legal Times. Only a reactionary could have been so discomfited by Savimbi's little cannibalism problem as to think this was not a daring contribution to the cause of Reaganism.
Again, Republicans have no special aptitude for corruption. Neither do Democrats. But it always comes, and it always (at least in healthy two- or multi-party democracies) leads to the end of the governing coalition.
NOTE: When I say "governing coalition," I don't necessarily mean the kind you get in parliamentary systems, where two or more parties explicitly share power in order to form a government. Yet even in our two-party system, we still have governing coalitions. The two-party system simply masks the fact.
The Republican Party includes everyone from Evangelicals to libertarians. The Democratic Party includes a similar range of interests. Each party is, in and of itself, a coalition of interests.