Obama doesn't need to be a drag for Tester, Nelson and maybe even Conrad (he was in the freinds of angelo thingy) to be in trouble. Also, McCaskil is in the mid 30's right now in approvals.
Better keep your eyes out for retirements in Nebraska, Florida, New Mexico, Wisconsin and Michigan. Currently the only opportunities for the Dems are in NV, MA and ME (Only if Snowe retires or gets primaried). West Virginia could join the list but it will be difficult to knock of an incumbent Republican in a state that Obama will be lucky to get over 43% in.
Incumbents dont lose in states their Presidential nominee carries unless they are involved in a scandal(Ted Stevens in 2008).
John Ashcroft did in 2000, granted there was a death involved.
In 1972 there were several Republicans who lost reelection.
A Republican incumbent, Larry Pressler I think, lost reeleciton in South Dakota to Tim Johnson in 1996 despite Dole carry the state. I don't rememeber a scandal just that the state grew weary of him. Sounds like something that could happen to Webb or McCaskil in 2012.
Here's how I see it based on the presidential numbers:
GOP 58% or better: GOP picks up NE, ND, MT, MO, FL, OH, and VA. MI, WI (lean GOP if open), MN, NJ and WV (if Manchin wins in 2010) are the toss-ups. Jodi Rell would make CT lean GOP and Pataki would make NY a toss up. All GOP incumbents survive, including Scott Brown. Even if a tea partier is running in ME, he/she would win the general.
GOP by 52-58%: GOP picks up NE, ND, MT, MO, and OH. FL, VA and WI (if open) are toss ups. Manchin would be slightly favored to win a full term in WV, and Scott Brown would be favored to hold on in MA.
GOP by 51% or less: GOP picks up NE, MT, ND, and MO. OH, FL, VA, and MA are toss-ups. ME is lean Dem if a tea partier beats Snowe. NV is lean GOP even if Ensign is the nominee.
Obama by 51% or less: GOP picks up NE, MT, ND, and MO. NV, OH, MA and FL are toss-ups. VA is lean Dem, and so is ME if the nominee is not Snowe.
Obama by 52-58%: GOP picks up NE, MT and ND. MO and NV are toss ups. NV is lean D with Ensign. OH and FL are lean D . VA is likely Dem (assuming Obama wins VA by at least his 2008 margin), MA is likely Dem, and ME would be likely Dem without Snowe.
Obama by 58% or more: The only scenario with net Dem gains. Obama +15-20 polls would be used to recruit top tier candidates in hard line GOP states. NE (assuming Obama would be winning NE outright), MT and ND are lean D. Every other Dem incumbent would be safe. NV would be lean D, ME would be safe D without Snowe, and MA would almost be safe D. Toss-ups: AZ, TX, and IN (with Lugar retirement), TN (with Bredesen) or UT (with Matheson), or WY (with Freudenthal).