It looks like Dems should gain, but four gains are tough, though possible.
Here's my quick review.
Democratic Seats (Two competitive)
California - Easy hold whether Boxer runs or not.
Colorado - A tossup race with Bennett the slightest of favorites.
Connecticutt - No money needs to be spent to hold Blumenthals' seat.
Hawaii - another easy hold.
Maryland - No competition for Barbara.
Nevada - I actually put Reid an underdog, tossup if he retires. If Sandoval jumps in, this becomes a Repub lean at least.
New York - Which Republican sacrificial lamb will agree to take on Schumer?
Oregon - Wyden is a lock.
Vermont - Leahy as safe as could be.
Washington - Murray seems to have a close race for a month, and then pulls away for a comfortable win.
Republican Seats (3 Dem favoreds, 6 more competitive)
Alabama - Shelby an easy win.
Alaska - Murkowski a lock unless VP pick.
Arizona - McCain retires rather than losing in primary. That makes this race a toss-up.
Arkansas - Boozeman a lock.
Florida - If Rubio runs, he wins. If he quits to run for prez, it's a toss-up.
Georgia - Isaakson probably cruises, possibility to challenge.
Idaho - Republican easy win.
Illinois - Kirk says he will run for reelection. Will be huge underdog unless some kind of weird health sympathy vote.
Indiana - Coats probably safe.
Iowa - Grassley safe. If he retires, a lean Dem. No Bruce, you can't run for Grassley's seat.
Kansas - Moran a safe bet.
Kentucky - Paul a lock. If he quits to run for prez, probably still likely Repub.
Louisiana - Vitter comfortable as a sleeping baby in diapers.
Missouri - Blunt a slight favorite - Lean Repub.
New Hampshire - Ayotte probably safe.
North Carolina _ Burr a tossup at best.
North Dakota - Hoeven safe.
Ohio - Ohio always close, but I'd make Portman a pretty strong favorite.
Oklahoma - no contest.
Pennsylvania - best pickup opportunity. Toomey a significant underdog to hold on.
South Carolina - Does this guy run every year? He'll win by 25 %.
South Dakota - Thune is safe.
Utah - Mike Lee is probably safe. His competition will be in primary.
Wisconsin - A good pickup opportunity. Ron Johnson can keep his seat but he'd start out a lean - Dem or maybe even likely Dem.