Power can corrupt someone, but it does not necessarily follow that it WILL corrupt someone.
(First point Personal Character)
As someone pointed out that absolute power, corrupts absolutely. I can think of one person, off the top of my head, that does not fall into that category. Queen Elizabeth of England. She is the most beloved and revered monarchs in English history. She was Queen in a time when that still meant something in the English government, and, by all accounts, her every decision was based on the benefit of the country and not of herself. In fact, she did not marry a man she reportedly loved because it could have damaged the english government. A little research in your local main library on monarchs should find several examples on both sides of the issue.
On the note of incomplete power, as is evidenced by America's Government... The President, nor any other office in the government has absolute power. I am sure that you have heard the term Checks and Balances. However, the minor power can go to some peoples heads. I believe this is more a question of individual character and morality than a blanket statement. If the character of the individual is such that they are focused more on what they personally can get out of any given situation, then it is more likely that they will be corrupted by any amount of power. However, if their character and moral beliefs are such that they focus more on what is best for others as a whole then they would be much less likely to succumb to degradation into corruption.
(Second Point - Public Opinion)
This is mainly a point that can be used in American Politics.
Although someone may be of a mind to sway toward corruptibility, if the majority of people will stand against any corruption, then the person will have no choice but to follow public opinion or risk losing their power. However, Just because someone does not follow public opinion does not mean they are corrupt. For example (and this is intentionally generic) , public opinion says that they want free use of sprokets, but it has been proven that sprokets are overly dangerous. An official that stands up against the use of sprokets is looking toward the benefit and safety of the whole (which kinda goes back to my first point but would make a good transition from one point to another).
(Third Point - Length of potential retention of power)
A dictator or monarch who will never (until death) be required to leave office is more likely to be corrupted than an official who takes office for only a small period of time. I.E. the fall into corruption, in most cases takes time, and the shorter the term of power, the less corrupt a person can become.
I understand that there will be exceptions to all of these items, but the topic of the essay is not a black and white, fact or fiction topic. There are grey areas involved that are based more on the person than on the power they represent.
I hope this helps, and if it does, I'd love to see how your paper turns out.