Double Standards

Whenever you make an argument, you are arguing for or against an unspoken standard. To say, “Politician X is dishonest” is to articulate the unspoken standard that “Dishonest politicians are undesirable.” If you then support Politician Y, who is also proven to be dishonest, then you have called into question your standard. By supporting Politician Y, and weakening that standard, then Politician X might still be dishonest, but you’ve made it clear that this doesn’t really matter anymore.