We are hardwired emotionaly to be a member of a human group and feel secure hearing what we want to hear from the membership. Once you belong, you will rationalise almost anything to maintain the comfort of your feelings and world view opposed to the pain and identity crisis incured in challenging ones own reality. This is why electorates are so hard on incombants who are felt to have dissipointed them but, like Robo Lemmings to those who keep them happy. Part of the reason McCain lost in 08 was all of the conservitive evangelistic electorate were dissipointed by Bush and enMass refused to vote because they saw McCain as Bush's 3rd term.
Right or wrong partisanship is a survival strategy evolved from being a member of a hunting band, tirbe, village, religion etc.
Our political affiliation and views are intertwined with our emotional need to belong. Not our rational choice to learn from past mistakes.