Nate Silver thoroughly debunks the "oversampling" charge thrown at pollsters by Republicans this week

In particular, he debunks the idea that the number of democrats in the sample is too high, because "party identification is not a hard-and-fast demographic characteristic like race, age or gender. Instead, it can change in reaction to news and political events ... Since changes in public opinion are precisely what polls are trying to measure, it would defeat the purpose of conducting a survey if pollsters insisted that they knew what it was ahead of time."