Public Policy Polling (pdf) (11/30-12/2, registered voters, 3/5-8 in parentheses):
Beau Biden (D): 39 (36)
Mike Castle (R): 45 (44)
Undecided: 16 (20)
(MoE: ±3.5%)
When PPP last looked at the Senate race in Delaware -- back in March, when the idea of Mike Castle getting into the race seemed kind of odd -- they found Castle with an 8-point edge. Not much has changed in the intervening months, even though Castle caught a lot of people off guard by making his entry official; Castle still holds a 6-point lead. This is a little more pessimistic than the most recent few polls (a 5-point Biden lead according to Susequehanna and a 1-point Castle lead according to R2K), but taken together, they average out to a very tight race.
Both have good favorables, with Biden at 43/35 and Castle at 55/28; the key number seems to be that Castle is cleaning up among independents (52-23), which helps him to overcome the Democratic registration advantage in Delaware. Obama has 53/41 approval, which doesn't seem particularly out-of-whack with where things stand nationally, but one strange finding is that the House health care reform bill had a net negative, 43-46, in this sample -- suggesting that Castle's vote may not have done him as much damage with this sample as it seemed to do with Susquehanna's sample a few weeks ago.
http://www.swingstateproject.com/diary/5974/desen-castle-has-edge-over-biden