What you describe is significant, but I would say is the minority of
politicians. What happens with most of them is both more banal and more
sinister.
Blackmail, not lust for power is the most common
motivator afaict. You're a new Congress critter, you're idealistic,
determined to make a difference. You attend your first cocktail party.
Someone must have put something in your drink, because you wake up in a
strange hotel room feeling strange and light headed. As you start to
get up, you notice a photograph on your chest. It is you in some very
compromising situations. There is a note on the back: "Respect the
blob. Learn from the Blob. Love the Blob. Or we release this and more like it."
If
you don't drink, they will find some other way to drug you. The only
defense is the truth - tell the world what happened. They probably
won't believe you, and you will lose your position and have trouble
getting a new job. But the more people have the courage to just tell
the truth, the less power these tactics will have.
There is another factor - empathy fatigue. This happens to other caring professions. After a decade of truly caring for patients/constituents, you lose the ability to care through sheer fatigue. It is time for a sabbatical - or a new career. This is the strongest argument for term limits IMO. Ron Paul is the only politician who seems to me to have fully retained empathy through his many years of service.