I've been watching the 2012 Senate shape up since November of last year. I'm a geek about this stuff I know. I have a dry erase board with all the races color coded Red-Blue and Green, with assessments of assorted races. Ranked by which will switch from Dem ->Rep.
In order for the GOP to take control of the US Senate they only need 3 or 4 seats minimum, (Depending upon the Pres. result), and up to 13 to have a filibuster proof control of the Senate. I read Chris Cilizza, and Larry Sabato, and several assorted people who really dig into these races. And I follow RCP Polls to gather Poll results and trends. A few of their writers Sean Trende and Jay Cost are great analysts. Dick Morris is also entertaining, albeit a little optimistic sometimes. The Dems are defending 23, seats the GOP only 10.
As of right now I see 11 races that are switchable, including the Scott Brown, MA race as the only GOP possible loss. Update 12/29 Brown is polling 6 points behind, but he came from 30+points behind in just a few weeks in the special election last year. (We are a year away and much can change). Also considering Mitt is the likely GOP candidate that will help Brown on the down ballot.
North Dakota This is a shoe-in.
Nebraska Ben Nelson and The Cornhusker Kick-Back are going down. Update 12/29 Nelson is retiring and this seat is now almost certainly a GOP pick-up
Missouri Claire McCaskill is a great Dem. Senator but she is still gonna lose. The Missouri comp.
Virginia No more Macaca..George Allen is taking this seat back that he barely lost. Tim Kaine will make it interesting though.
Montana John Testor just barely won this in 2006, he will have a difficult time holding on in 2012, with new TEA Party momentum.
Wisconsin Herb Kohl's retirement should make this a slight GOP favorite depending on the candidate.
New Mexico Open Seat..should be winnable by Heather Wilson over Dem's Heinrich or Balderas.
Ohio Sherrod Brown will not go away easily, but what I have learned about Josh Mandl should be a good race.
Florida Bill Nelson will be a tough race for the GOP to pick up but still in range.
Michigan Debbie Stabenow will be tough to defeat as well. But if Pete Hoekstra is the Candidate and Mitt Romney is the GOP Nominee..she will be in trouble.
So If the GOP runs the table on the above Dem held seats the pick up is only 10. In order to get to 13 lets look at the rest of the Dem held seats, that are remotely possible:
CA: Feinstein Highly unlikely
CT: Open seat Slim Chance, Linda McMahon is in and so is Chris Shays..With Romney on top possible pick up.
DE: Carper Highly unlikely
HI: Case Highly unlikely, Best chance is if Former Gov. Linda Lingle runs. Update Lingle is running so there is some hope on this one.
MD: Cardin Highly unlikely.
MN: Klobuchar Some Chance. If Michelle Bachmann had jumped into this race the day she won the Iowa Straw Poll, this would've been hers for the taking. A Tim Pawlenty run would also be formidable!
NJ: Menendez No Chance..So far I don't even know if he has a challenger.
NY: Gillibrand No Chance
PA: Casey Some Chance, but if Rick Santorum got in this race, it'd be 50/50.
RI: Whitehouse Highly unlikely.
VT: Sanders Slim Chance. I need to learn a bit more about Tom Salmon.
WA: Cantwell Slim Chance. Wait to see how the GOP Primary turns out to get a gauge.
WV: Manchin Possible. Shelly Caputo Moore is a well known candidate in WV, in a perfect storm this state could fall. But Joe Manchin is the most Conservative Dem on this list, he could even switch parties after the election if he somehow was the 60th vote.
So As I see it maybe 10 pick ups which would be huge!! But it still wouldn't be 13. The class of 2014 might be so intimated that a few party switches could happen, like in 1994. AK, AR, CO, LA, NM, VA, NC Senators could be trembling, and might at least consider voting to repeal Obama-Care if it is still required by then.