It seems like just about every cycle there's a Senator who a lot of noise is made about the vulnerability of who turns out to be just fine. For the 2012 cycle I think that's Bill Nelson.
Yes Nelson's approval numbers are under 50%. But that's because Democrats aren't in love with him. Earlier this month Quinnipiac found his overall approval at 45%. That's because only 55% of Democrats gave him good marks. At the same time though he had 37% approval from Republicans- well above average- and also 44% approval with independents, twice the number that disapproved of him.
Those trends are very similar to what we found for Nelson the last time we polled on him in mid-December. We had him at only 36% approval, with 33% disapproving. That low topline approval number was largely due to his being at only 45% with Democrats, but there too we found him at an unusually strong for our polling 23% with Republicans and above ground with independents at 42/36.
Here's the thing about Nelson's low numbers within his own party- the overwhelming majority of Democrats who don't approve of him are going to vote for him next fall. Nelson doesn't have terribly high disapproval numbers with Democrats, just low approval. Confused? It's because a very significant portion of Democrats are ambivalent toward him- they just don't know enough to have an opinion about him either way. But they're sure going to go vote for him while they're out to cast their ballots for Barack Obama next fall. And when he combines the support of his base with his unusually high crossover support from Republicans and solid standing with independents he's likely to get reelected unless next year turns out to be an exceptionally bad one for Democrats.
One other note- the magic 50% rule ain't that magic and that's especially so in a big state like Florida. It's a lot harder for politicians in large states to hit 50% approval or reelect number because voters just don't tend to know their officials as well as in smaller states where there are fewer tv markets to show up on and cities to appear in. There's no reason to think voters ambivalent toward Nelson will end up breaking away from him because he's an incumbent. They're much more likely to end splitting their votes pretty evenly.
I'm going to do a list of the Senate seats most likely to flip once we're done with our polling tour of the 2012 US Senate races and Bill Nelson will not be terribly high up on it- I think the Sherrod Browns and even Debbie Stabenows of the world may end up with more trouble than him.