It is frequently said that people rarely change their minds about politics because they insulate themselves from information that contradicts their point of view. According to the Oxford Handbook of Political Psychology, however, recent research shows that this is not true. People don’t insulate themselves from contrary points of view, they just subject arguments against their worldview to much harsher criticism than arguments favorable to their worldview.
This is why presidential campaigns result in very little overall persuasion. That doesn’t mean that there is no point in having the campaigns, though, because they serve a useful reinforcing effect for people who agree with the candidates. People need to know that there are other people who agree with them.