Let's keep in mind, too, that the Democrats have had a tremendous advantage since the days of FDR, because the media and intellectuals have basically sided with them. As a result their crimes generally have to be more flagrant before they are punished.
When you think of the various political machines in the U.S., in which unpunished corruption has run rampant, there are a few famous Republican machines, but I'd venture to guess that 90% of them are Democrat or populist. The Nassau County Republicans were one of the most famous exceptions, and they finally imploded.