Mason-Dixon for Ron Sachs Communications (8/18-22, registered voters, 2/9-10 in parens):
Bill Nelson (D-inc): 49 (49)
George LeMieux (R): 34 (35)
Bill Nelson (D-inc): 45 (46)
Adam Hasner (R): 34 (24)
Bill Nelson (D-inc): 44
Allen West (R): 38
Bill Nelson (D-inc): 45
Vern Buchanan (R): 35
(MoE: 4%)
What's interesting is how little this race has changed since February. The name recognition of Florida Republicans (if we take Adam Hasner as our proxy) is climbing, but ever-so-slowly. I'm also intrigued by the fact that against George LeMieux specifically, Bill Nelson does the best. I can only guess that some portion of Floridians who are otherwise undecided about Nelson realize they dislike LeMieux so much that they're ready to vote Democrat. Usually you might expect this kind of behavior when the challenger is some kind of extremist, rather than someone with a mostly moderate background, but I guess "careerist suckup to Charlie Crist" isn't exactly a great political profile either.
You'll note as well that Mason-Dixon test one guy who has already said he won't run (Allen West) and one guy who has shown very little interest in running and says he won't decide until later this year (Vern Buchanan)—not that either of them do particularly well against Nelson. Of course, Nelson himself is under 50 percent, but if there's anyone who can convert the necessary undecideds, it'd be him.
M-D also surveyed the GOP primary field:
Allen West (R): 11
George LeMieux (R): 7
Adam Hasner (R): 8
Vern Buchanan (R): 14
Craig Miller (R): 5
Mike McCalister (R): 2
David Gee (R): 1
(MoE: 5.8%)
These guys have a long way to go.