At the same time, elections are in about 3 weeks - important elections. Most people who have been paying attention until now had already decided who they were voting for a long time ago. But the people that swing the election are often just tuning in now. Early prediction (even though I hate predictions, which tend to be meaningless): The GOP hangs on to their majorities everywhere, shocking and angering the left whose polls told them they'd take the House and come close in the Senate. People forget that even a 50/50 representation of Republicans and Democrats isn't necessarily an accurate representation: There may be more Republicans than Democrats in that area. Throw in that the polls are actually taking Democrat-heavy samples much of the time, and it's no surprise that the polls are consistently off.
On top of that, polls tend to call people during the day. (At least, based on when I've gotten calls.) Well, who is home during the day? Retirees/older people; young people who are in school [school is not all day]; unemployed people; people whose jobs allow them to be at home; stay at home moms. More importantly, who is NOT at home? Working people, businessmen, etc. Which grouping is likely to vote Democrat? Women tend to lean Democrat (whether because of abortion or other reasons). Older people do (Social Security). College students do (brainwashed). Unemployed people do (welfare). Finally, Republicans are more hestitant to trust pollsters, so they turn them down more.
If you look at the bottom of a poll, it generally shows the makeup of the people polled and when they were called. The percentages are incredibly off from what a typical population should be and actually are. The +/- 3% they generally give are too small: I'd say that any race that's within 10-12 points (led by a Democrat) going into election day is winnable for a Republican (though not necessarily going to win). On the flip side, I'd say that a 6% lead by any Republican is safe enough.
If I can, I'll try and test this in Novem