Those with potentially competitive races: (The higher the rank the more competitive it will be)
Those almost certain to lose:
-Nelson and McCaskill have virtually no chance unless their GOP opponent implodes. Brown, Stabenow and Tester are in a bit better shape but all should have decent opponents (especially Stabenow) and Ohio and Montana should roll to the GOP in the Presidential election which will help down-ballot Republicans.
Those in great trouble:
-Klobuchar will be helped by Minnesota's record of voting for Democrats in Presidential elections (they haven't voted Republican since 1972) and the GOP will have to come up with a good opponent. The Gopher State has elected two Republicans to the Senate since 1994 so it is quite possible they'll elect another one. Webb hasn't decided if he'll retire but even if he doesn't he'll be a long shot to win again. If he does retire that seat becomes a solid GOP pickup. Nelson is in great danger. It is not out of the realm of possiblity that Obama will lose the state by 15 points which would sink any down-ballot candidate. Nelson's a liberal who pretends he's a moderate and this will be fully exposed in the campaign. Bob Casey is a bit more moderate and the state is blue when it comes to Presidential elections. It hasn't voted for a GOP candidate since George H.W. Bush in 1988 (although his son came close in 2004). This will be ground zero for the Presidential race. If Obama can't hold the Keystone State his chances of reelection are virtually zero so he'll pour money and volunteers into the area which should help Casey. If the economy continues to fizzle and the GOP can come up with a strong opponent it won't matter what the Administration does, Casey will lose.
Those who could be in trouble:
-There's no reason to think Kent Conrad couldn't face a tough election. North Dakota will easily fall to the GOP candidate for President. And, like Nelson of Florida he's a liberal who pretends to be conservative. Byron Dorgan, his fellow Democrat from the state bowed out of the election last year when he faced an almost certain defeat. Menendez is in a stronger position because New Jersey is so blue. Moreover, the GOP has yet to find someone who will be willing to run such an expensive campaign. Maria Cantwell and Ben Cardin are in similar positions; they would only lose in their blue states if the GOP put up strong candidates. The GOP has a better chance in Washington; after all Cantwell barely won her first election in 2000, winning by a little over 2,000 votes. Moreover, Patty Murray won her last election by only 2% last November after winning handily in 2004. If Murray can be put on the ropes Cantwell is vulnerable too. Maryland is so deep blue it would be hard to beat Ben Cardin but Mike Steele gave him a tough run in 2006. However, the GOP bench is small. I don't know if Steele would be willing to run again. The only other alternative is the former governor, Bob Erlich. Joe Manchin faces an interesting dilemma: should he run as a Republican in 2012? West Virginia will give between 60-65% of its votes to whomever the GOP Presidential candidate is. Would any Democrat want to run down-ballot in that environment? We'll see. Herb Kohl hasn't had a tough race since he was first elected in 1988 but his negatives are high this year. I think he may retire. He hasn't done anything in the Senate of note and I don't know if he really cares to be there anymore. Gillibrand will benefit from being in New York but would have a serious challenge if George Pataki, Rudy Guiliani or Peter King ran against her. If any of these three men jumped in I would immediately make him the favorite to win the full six-year term.
Those who are safe:
-Tom Carper is in good shape in Delaware. There's no GOP candidate who can match him-he's just too tough. Sheldon Whitehouse will run for reelection in one of the bluest states in the Union. The same goes for Dan Akaka and Diane Feistein. The only way the GOP could compete in California (or New Mexico, Vermont and maybe Hawaii) would be if the Democratic incumbent decides to retire-which they may do.
-It is simply hard to say what he'll do. Certainly the Democrats want to elect someone else. Will he become a Republican? Will he run as an Independent? Will he retire? His negatives are much higher than they were in 2006 so he's in a tough spot. We'll just have to wait and see.