The Most Vulnerable Republicans

Right now it seems unlikely that any incumbent Republican Senators will lose for reelection this fall- just as no incumbent Democratic Senators lost in their party's strong years of 2006 and 2008. But with the voters in an increasingly anti-Washington mood it's always possible incumbents of both parties could be in some trouble this fall- and of course it's also possible Democrats could be in much better shape eight months from now. With those things in mind here are the four most vulnerable incumbent Republican Senators, even if they're all favored for reelection as of now:

1) Richard Burr of North Carolina. Polling from across the ideological spectrum has put Burr's approval rating in the mid-30s and Democrats have a candidate in Elaine Marshall who's shown the ability to win statewide and in Cal Cunningham who has a good story to tell and has been victorious in a Republican leaning area before. Burr leads right now by margins similar to what Elizabeth Dole had at this time in 2008, and she ended up losing badly to a candidate who had begun the campaign with virtually no name recognition.

2) David Vitter of Louisiana. Although recent Rasmussen polls have shown Vitter with a healthy lead, Charlie Melancon is a top tier challenger who though not yet known statewide has performed strongly in a very tough district for Democrats. Voters have had a long time to forget Vitter's past personal transgressions but they'll be reminded of them this fall. Louisiana is also a state that might actually be easier for Dems in a midterm because it's one of the few places that supported Barack Obama at a lower rate than John Kerry.

3) Johnny Isakson of Georgia. Isakson has worse approval numbers than Vitter at a 36/38 approval spread but ranks lower on this list simply because he doesn't have a serious Democratic opponent yet. At this time two years ago no one thought Democrats had any chance at Saxby Chambliss, but they pushed him into a runoff even with a pretty uninspiring candidate in Jim Martin. This is an opportunity for an ambitious Democrat to, worst case scenario, get ahead in line for a future statewide campaign by putting up a strong performance in a tough election year and best case scenario, pull off a shocker.

4) Jim DeMint of South Carolina. Although there aren't very many voters who dislike DeMint, there are also a surprisingly large number with no opinion of him, putting his approval well under 50%. We found him at 44/29 in December and a Winthrop poll last week put his numbers at 43/28. When you are undefined to 30% of the electorate after five years in the Senate that gives someone else a chance to define you and DeMint's gallivanting around the country could prove to be a liability if voters don't think he's been attentive enough to South Carolina. But like in Georgia, Democrats will need a candidate if they're going to take advantage of these numbers.

I was going to make this a top 5 list, but the other Republican Senators up this year that we've polled on- John McCain, Lisa Murkowski, John Thune, Tom Coburn, and Richard Shelby- all look completely unbeatable. And I haven't seen much evidence that the three we haven't polled on- Mike Crapo, Chuck Grassley, and Bob Bennett- have anything to worry about.