Dan Ariely conducted an experiment where he asked participants to rate the acceptability of a candidate indulging in "ethically gray activities" in order to get elected:
What we found was that participants who were planning to vote Democratic indicated that Romney should be held to a fairly high ethical standard. Republican participants held a similar standard for Obama. But when participants happened to support the candidate in question – whether Obama or Romney – they indicated that ethically gray activities were approximately 3 times more acceptable.
If voters hold a double standard for the ethical conduct of their own candidate and the opposing candidate, the overall standard of ethics is likely to fall to the lowest common denominator.