The wise aphorism about power has of course become a cliché. The less discussed corollary is that power attracts the corrupt, and the corruptible.
Sometimes you do hear the saying "those who most want power are usually least deserving of it" which is somewhat similar, but I think that saying is normally understood only in reference to people seeking the very top jobs. What I'm getting at here has more to do with the commonly observed rot that seeps into any political system that sees the same group in power for too long. This is where you often get scandals that often don't originate at the top, where you may not have a Nixon type charater purposely running a kleptocracy, but instead you have just a seething mass of opportunists and schemers, coupled with lots of careerists who won't stick their neck out to stop the corruption around them, even if they usually don't initiate it. What often explains this to me, is that the type of people who are already amoral and corrupt, or who have a weak sense of ethics and plenty of ambition are the type of people who are actually fairly non-ideological and will just pursue the path of least resistance to be at or near power. This isn't just the politicians themselves, but the consultants, aides and other background types who may not have the particular attributes needed to succeed in office directly, but can still reap benefits of influential less visible roles.
In a stable political system dominated by some particular faction, the obvious way to attain power is to just join that faction. After all, why join the hopeless Rebels when you can more easily join the Empire? Even if you privately have beliefs more in accord with those out of power, you'd probably rather jump on the existing gravy train while the getting is good. This sort of thing goes a long way to explaining the phenomena of Democratic big city machines, which just aren't really ever able to clean house, since voters will reliably return Democrats to office far too often, despite obvious signs of infestation.
This isn't an ideologically specific problem though. I do think right wing parties are more amenable to corruption if only because right wing ideology is of course most compatible with sociopathy to begin with (though their penchant for strict discipline and harsh top-down control may provide some counterweight). If you're already a sociopath, and current best estimates say about 1% of any given population is, and you could join a group dedicated to bringing about a more equal world, or a group dedicated to letting the powerful accumulate as much as they possibly can, which would you naturally gravitate to? In which would be easier to hide your true nature? Anecdotally (and of course we will have no prospect of real data on this since it's hard enough to study sociopaths in normal circumstances, never mind powerful and famous ones) it isn't difficult to find the obvious sociopaths in the ranks of the right, from Gingrich divorcing his wife while she was in hospital for cancer, Rove's brand of anything-to-win ratfucking, Ailes' shameless propagandizing to the various now infamous grifters and liars like Delay, Abrhamoff, Nixon, North, and Norquist.
This ties into the oft-noted non-equivalence where there simply is no left wing version of Hannity, Coulter, Palin, Beck, etc etc. As Jon Stewart once said, liberal hacks aren't nearly as effective as their conservative counterparts, because you can tell they feel shame when spinning some awkward or indefensible position.
But still, no left of centre party or movement is immune to drawing in various corrupt and corruptible individuals once they actually attain and maintain power. Certainly no one thinks the various big city Democratic machines are clean operations. It's a real problem for those concerned with the very systems and structures of politics. No answers necessarily but awareness is still of some value.