Fred Barnes notes that it is theoretically possible for the Democrats to regain the Senate even while Bush wins re-election–if Daschle also wins re-election, Democrats lose only two races among the five in Georgia, the Carolinas, Florida, and Louisiana, and the Democrats pick up all four of the vulnerable Republican seats in Alaska, Colorado, Illinois, and Oklahoma. Everything Barnes says is true, but he doesn’t say whether he thinks that a Democratic takeover of the Senate is more likely than an increased Republican majority. I think just looking through the conditions listed above suggests that Republicans are still likely to pick up seats.
I would also add that even if the Democrats net two seats, in their dream scenario, conservatives will have the same number of solid votes–the two gains in the South will neutralize the losses in Illinois and Oklahoma. (Indeed, people who are tougher on Senator Fitzgerald of Illinois than I am may consider that a pick-up.)