Although Republicans thought Nate Silver was biased towards President Obama, he ended up enhancing his reputation for accuracy in hte 2012 election. Silver has a nice column up today that examines whether the polling data supports the theory that Obama successfully defined Romney as a greedy corporate drone that did not care about average people by buying a large number of ads in the spring and summer. The data does not support that interpretation.
People love to create appealing explanations for things that happen, but that does not make them true. Even if you do not know the correct explanation, you can frequently test the ones you have. It is called the scientific method.