Hint -- the Siena poll shows it's not Israel, as Gawker reports:

One surprise in Siena's polling, however, is the relative importance of Israel to the race [...] according to the numbers, it may be less of a factor than it seems.

"I don't see it at all," Greenberg said, when asked about the "Israel effect."

Siena asked voters to pick from five options to explain their vote, including the candidate's party, position on Social Security and Medicare, whether they were endorsed by a trusted source, their position on economic issues, and finally their position on Israel. Only 7% of voters picked Israel, including just 16% of Jewish voters.

That may not be the whole story, however: Weprin's lead with Jewish voters has collapsed from 21 to 6 in the last month. It's roughly in line with the total 12% drop among voters overall, but may be more complicated to tease out. Jensen, for his part, doesn't want to make any conclusions on the Israel issue without seeing more detailed results first.