The Cook Political Report explains (subscription required):

Now that Bayh’s seat is open, we moved the race from the Lean Democratic to the Lean Republican column. As a result, we now rate eight Democratic-held seats either in the Toss Up column, or tilting in varying degrees toward Republicans. The open seat in North Dakota where Sen. Byron Dorgan is retiring is in the Solid Republican column as Democrats struggle to recruit a candidate who can compete with popular GOP Gov. John Hoeven. The open seat in Delaware, which is a special election to finish the remainder of Vice President Joe Biden’s Senate term, is now in the Likely Republican column. There are five Democratic-held seats in the Toss Up column: Sens. Blanche Lincoln in Arkansas, Michael Bennet in Colorado, Harry Reid in Nevada, and Arlen Specter in Pennsylvania, as well as the open seat in Illinois. Sen. Barbara Boxer in California and the open seat in Connecticut are in the Lean Democratic column, bringing the total to 10 seats.

Getting to 10 and flipping control of the Senate is a bit dicier, and Cook cautions that to do that, Republicans would have to put more seats in play, avoid flaky primary choices who won’t play well in the general races, improve fundraising, and maintain the political momentum they’ve been building. The bottom line: “For now, while it is theoretically possible for Republicans to gain the 10 seats they need to win a majority, it remains a very difficult task.”

With so many seats in play, the question remains how this will affect Senate Democrats in the run-up to the November elections. If Obama has his way, they’ll double down and push through his agenda. But nervous incumbents can see the trends and read the polls. For those who still have a fighting chance, the trick will be to distance themselves from their prior voting records, show they’ve heard the voters, and cast some votes that demonstrate independence and fiscal sobriety. That, however, means resisting the entreaties of their leadership and managing to get votes on legislation that will help them.

It’s not clear that incumbent Democrats who have voted in lockstep with the the Obama-Reid agenda have the moxie or skill to do that. In fact, Kirsten Gillibrand and Michael Bennet joined the double-down crowd by casting their lot with not only ObamaCare but also the jam-through-on-50-votes strategy (i.e., reconciliation). That seems certain to make their precarious situations even shakier.

Democrats might retain a bare majority, provided they stop voting for legislation their constituents hate, Obama’s popularity rebounds, and unemployment begins dropping. Not all that likely? Then you can conclude that control of the Senate really might slip from the Democrats’ grasp.