Predictions of who is going to win a political campaign are fascinating stuff. Many people project their own preferences onto an outcome. In the literature that's called wishful thinking. The origins of wishful thinking research is out of studies of sport fans who think their team is going to win even when all the evidence suggests otherwise. Then the notion got moved over into politics. A few studies have been done. Here is some my own data.
Respondents were asked who is going to win. I then combined that with who they supported. Looking at presidential elections from 1968 to 2004, you see how much preference guides prediction.
Republicans: Out of 6,960 surveyed, 81 percent predicted their preferred candidate would win.
Democrats: Out of 6,664 surveyed, 64 percent predicted their preferred candidate would win.
The amount of wishful thinking from year to year is fairly stable, but predictive accuracy (getting it right) ranges wildly from election to election. This is important. When elections are close (like 2000), personal preference and wishful thinking gets in the way and people are less accurate in predicting the outcome.
When it's a runaway election, like 1972, then accuracy improves. You'd think media exposure would improve accuracy, if for no other reason than reading or watching the news would expose people to public opinion polls and the viewpoints of others, but it does damn little to dampen the preference -> prediction bias.