Republicans have been loaded with criticism on poll party ID. As is typical with the media there's no real evaluation of Republican critique, simply a dismissal that Republicans are crazy and only listen to Rush and watch Fox News. Of course I haven't seen anything on other news sources to explain this enivornment.
Above are a summary of polls from the two weeks ending last Friday, September 21. The top numbers are exit polling party ID in the last four elections. The party ID for recent polls is below. Highlighted in yellow are party ID samples which are out of the range established in the last four elections. In 19 polls, there are more Democrats than there have ever been. In 11 polls there are less Republicans than there have ever been. In one poll there's less Democrats, but that poll also has less Republicans.
When you look at voter turnout there are always a cause and effect. There are usually major reasons one party is popular/unpopular and the party faithful are excited/disillusioned. An unpopular President can invigorate the opposition and disillusion the party, as George W. Bush did in 2008. I don't think Democrats are disillusioned, but the polls reflect Republicans being more disillusioned and not voting than 2008. A lot more.
What these critics never address is whether they really believe Virginia will only have 24% Republicans and explain why Republicans would be 9% below anything we've ever seen. Their answer on why this year is more Democratic than 2008 is, "'Cause it is."
Edit: Here Jim Geraghty provides evidence that this year definitely should be more Republican than 2008.
2nd Edit: Geraghty is asking the same questions I am.