Accuracy of State Polls

Something to keep in mind:

The first thing that jumps out at you if you read the state polls, is that there are a lot more polling groups doing polls at the state level, than at the national level. Also, most of the polling groups which do national polls, do not also do state polling, probably because it is expensive and difficult to try to cover all of the states on a consistent and timely basis. I have written before that national polls often focus on urban centers, which means that many of the states would require a functionally different methodology to work than what is used nationally. State polling tends to be smaller in respondent pool size, smaller in budget, and less frequent. Some polling groups only do one poll for the whole campaign, and it's common for even major groups to do a poll only once a month....

2006 was a bad year for republicans... it's not surprising that democratic party supporters gained a few points (usually 1 to 3 points) relative to 2004 in voter participation. So I went back and looked at voters by party affiliation, and compared those balances to this year's weighting by Survey USA. In thirty-six states, the party affiliation weights for democrats used by SUSA was five points or more higher than in 2006, a high-water mark for democrats. In twenty states, the party afiiliation weights for democrats used by SUSA was ten points or more higher than in 2006, and in eight states, the party affiliation weights used for democrats by SUSA was thirteen points or more higher than in 2006. Significant battleground states affected by this bias are as follows:

Pennsylvania: D+5 in 2006, SUSA using D+19, 15 point variance
Indiana: R+14 in 2006, SUSA using R+1, 13 point variance
Nevada: R+7 in 2006, SUSA using D+6, 13 point variance
Colorado: R+3 in 2006, SUSA using D+9, 12 point variance
Iowa: R+2 in 2006, SUSA using D+10, 12 point variance
Virginia: R+3 in 2006, SUSA using D+9, 12 point variance
Ohio: D+3 in 2006, SUSA using D+13, 10 point variance
Missouri: R+1 in 2006, SUSA using D+7, 8 point variance
North Carolina: R+1 in 2006, SUSA using D+5, 6 point variance

I've looked at the publicly available records on historical election participation, 2008 new voter registrations, and the Census information on these states, but I can find no valid reason for such large and arbitrary changes in political affiliation weightings. I would therefore submit that the models being used for many of the state polls have design flaws, which threaten the credibility of their published results.

Source.