Nate Silver is currently projecting that Democrats will lose 6 to 7 seats in the Senate. I actually think that political forecasters are currently over-favoring Democrats in the coming mid-terms. No, not because they're all biased and dishonest, but because they naturally work with the data they have: polls and demographics.
Working with the data I have, on the economy, I think those polls are only going to get worse. The economy is at best heading into a soft patch; at worst it is going into the second dip of a double-dip recession. That means that in the fall, as voters sharpen their focus on politics, they will be sharpening their focus on the economic conditions, and the politicians who haven't fixed them. Those conditions will at best be unimproved, and at worse reflect lengthening unemployment and financial disaster as credit lines are finally exhausted.
As I've said before, I'm not arguing that this is fair--I don't think that Democrats (or Republicans) are responsible for the current crisis. And the current Republican party is not currently going in a direction that would make me thrilled about them taking over Congress. Though of course, neither is the current Democratic party. I'm hard to please.
In the end, I doubt that Republicans will take over the Senate, simply because they'd have to run the table, and that doesn't seem very likely. But I'm pretty sure they'll take the House, because the polls that forecasters are using now seem almost certain to get even worse in the months ahead.
Naturally, all the usual caveats: forecasting is hard, and I could be surprised. But one thing is nearly certain, which is that unemployment is a lagging indicator; no matter what, it is simply not going to improve much by the time of the election. And unemployment drives some of the worst economic problems that people focus on (including, in many cases, foreclosure). Given that, I don't see how things do anything but get worse for Democrats in the fall.