Out of how many candidates were you selecting? 300 million? Or just a few, with long political careers? You don't get to the point of being an actually electable candidate if you could stir up a real mess within the establishment.
The entire political process is a vetting system to weed out the unreliables and the ones who won't play by the rules.
Also, I have an inkling that the system is a sociopath pass filter; at any level of government the candidate who is slightly more immoral has improved chances. After a few iterations we have power lusting vampires in charge (the Cheneys, Clintons, ect.)
