Trende, who writes the horse race blog at RealClearPolitics is among the best in country, and here is his take on various polling data from the MA Senate race:

Consider: PPP (D) has her at 47%. The Globe/UNH poll has her at 53%. Rasmussen has her at 50%. Even internal Democratic polling — which usually represents the Democrat’s best-case scenario — has her leading state Senator Scott Brown 50%-36%. In other words, most of the variance comes from Brown’s numbers — which vary between 36% and 48% support — not Coakley’s. As I’ve noted before, when you see one candidate very stable and one candidate with a high degree of variance in their numbers, it means that the undecideds are trending toward the candidate with the higher variance. In other words, that candidate will tend to be toward the high end of their polling range.

This is where the “undecided rule” starts to come into play. It’s a political science rule that predicts that undecided voters will break heavily for the challenger.”

Sean Trende also discusses the new-internet age “undecided rule,” and up-dates it. Here is his bottom line:

“So if we look for a principle that survives this new age of saturation advertising and internet-driven intensity, we might say that when you have two well-known candidates, the undecided rule is probably inapplicable as a predictive device. But if there’s a disparity between the candidates, the undecideds will still tend to break toward the lesser-known candidate. There’s probably caveats and exceptions here, but I think that’s probably about right.

“So what does that tell us about the Massachusetts Senate race? We have a sitting Attorney General who came out of a contested primary, going up against a more-or-less completely unknown state Senator. She’s struggling to get above 50%. All of this points toward a very close final race — potentially much closer than a week ago when I guessed at a 54-46 spread. Again, this is also consistent with what we’re seeing in the variance in the Coakley/Brown numbers. Coakley should be worried.”

The Dems keep thinking that the health care jihad has no consequences. How wrong their jihadists in chiefs: Pelosi, Reid and Obama are.