In a poll commissioned by Daily Kos and conducted October 20-22, Research 2000 found 10th District Democratic challenger Dan Seals leading GOP Rep. Mark Kirk by six points, 49-43 percent.  From Markos' write-up:

Last time we polled this race, the Kirk campaign absolutely freaked out, accusing us of being anti-semitic because we (admittedly) polled during Rosh Hashanah in one of the most Jewish districts in the country. Their argument was that it was our dastardly evil plot to disenfranchise Jews, which was weird since Jewish voters are some of the most loyal Democrats in the country. Our theory was that we had undercounted Seals support in that first poll because of its timing, but Kirk saw it differently.

Hilariously, a few days after our poll, SUSA came up with their own, and that one had SEALS up 52-44. Suddenly, the Kirk campaign loved our poll, and used it to push back against the SUSA one which, unfortunately for Kirk, wasn't conducted during a high holy Jewish holiday.

Neither was this one. So what has changed? Undecided voters. Seals went from 70-12 among Democrats to 83-10.