The double-digit lead is as believable as Sestak's lead...
There isn't a double digit lead now but he clearly had one (or close to a double digit lead) a few weeks ago confirmed by basically every poll.
My point is:if you could trust the 8% lead before based on polls alone,why wouldn't you believe that now it's basically a tossup?
Because there wasn't any reason for the shift. Such a major shift doesn't happen without a reason.
Obama rally in Philadelphia, Democrats coming home in the final weeks of the campaign, some campaign ad resonating. It could be a lot of movement among fickle undecideds rather than voters actively shifting from Toomey to Sestak.