That's why democracy works. It is natural for humans to strive for power. Its happened in history millions of times. Democracy works to use ambition, instead of fighting against it. Let us say there is a representative who wants to be president. How would he accomplish this? By being a star representative, doing the best he can to earn the love and respect of the people. So democracy harnesses the natural greed and ambition of individuals and forces them to use their goals to help out society as a whole.
There was a point made in an article I read a while back that was put well enough that I'm just gonna quote it.
"Saying "power corrupts" is stating something so obvious we feel stupid even typing it. It's like saying elevators elevate. If you found out tomorrow your congressman was caught firing orphans out of a cannon, you'd barely raise an eyebrow.
The thing is, it's the desire itself that's poisonous. You find that need for power most in the type of person who hates having to obey all of society's social contracts, particularly the ones that require them to not act like cocks all day. These are the people who are only nice guys because of fear of retribution if they do otherwise, so their main goal is to become strong enough that no retribution is possible (this is why sociopaths tend to seek positions of power, by the way).
So it's not just that power will destroy you. It's that the urge itself is bad news. That desire for power is a vicious, ravenous animal and feeding it only makes it strong enough to tear its way out of your belly and go on a bloody rampage." (http://www.cracked.com/article_17061_reminder-5-things-you-think-will-make-you-happy-but-wont.html)
Basically, power doesn't change or corrupt people, it just allows people to act the way they have always wanted to.