Susquehanna Polling and Research (R) (11/10-15, registered voters, 4/27-30 in parentheses):
Beau Biden (D): 45 (34)
Mike Castle (R): 40 (55)
Undecided: 8 (8)
(MoE: ±3.4%)
Mike Castle may be rasping to himself, Lloyd Bridges-style, "Looks like I picked the wrong week to run for Senate." And Beau Biden, who's been strangely coy about whether or not to run for Senate since his return from Iraq, may suddenly feel motivated to declare his intentions.
Susquehanna does a fair amount of Republican internal polling, so unless they're trying some weird messing-with-Biden's-head psy-ops, that makes the finding of a 5-point Biden lead all the more surprising. The last time they looked at the race was April, when Castle had a 21-point lead instead. Between this and R2K's findings last month that the race was a dead heat, it may be time for the rest of the punditocracy to re-evaluate whether this is really the GOP's best shot at a pickup.
UPDATE: The full memo is available now, and Susequehanna attributes this shift in large part to Castle's "no" vote on health care reform, which occurred shortly before the poll went into the field and apparently didn't play well with the state's left-leaning electorate. They also point to Castle polling better among Republicans (72-17) than Biden does among Democrats (65-21), but given Delaware's sizable Democratic registration advantage and a narrowing Castle edge among indies (42-37, down from 55-28 in April), that's enough to put Biden over the top.
http://www.swingstateproject.com/diary/5910/desen-biden-up-5-on-castle