Barack Obama's approval is clearly on the decline and there's been a lot of bad polling recently for Democrats related to 2009 and 2010 races but one thing that doesn't seen to be changing concurrently is the nation's party identification breakdown.
If you look at this handy chart pollster.com keeps of party id among registered and likely voters, it's basically flat since January. Overall Democrats have a 38-31 advantage over Republicans, almost unchanged from the 39/32 spread exit polls showed nationally on election day in November.
So if there's no party id shift but Dems are polling poorly, what's going on? Two things:
1) Independents aren't reacting as well to Democrats now as they were a year or six months ago, which is not surprising. They'll be back with us this time next year if there's significant progress on the economy and if not they probably won't.
2) Republicans are getting hyper partisan. We are frequently finding approval ratings of less than 15% among them for Obama and some Governors and Senators in our state by state polling. It's usually not that low.
Given the fact that there hasn't been a major shift in the partisan landscape Democrats will be fine next year if they can get their folks out and get back to the break even point with independents. Of course that might be easier said than done- sure isn't happening in New Jersey and Virginia right now.