NOM Founding Chairman Prof. Robert George explains at the Mirror of Justice blog:

In the run up to the election, a group of Orthodox rabbis, most from Brooklyn, but including others, notably Rabbi Shmuel Kamenetsky and Rabbi Simcha Bunim Cohen, two nationally prominent Orthodox Jewish authorities, published a letter stating that "it is forbidden to fund, support, or vote for David Weprin." The reason? As a member of the New York state legislature, Weprin, despite his Orthodox Jewish beliefs, voted to redefine marriage to include same-sex partnerships. This, the rabbonim declared, was chillul Hashem---a desecration, or bringing of shame, on God's name. The rabbis went on to say that "Weprin's claim that he is Orthodox makes the chillul Hashem even greater.

The letter from the rabbonim went farther [...] by asserting that under Jewish law "it is incumbent on every Jew" to support and vote for Weprin's opponent, "if the opposing candidate is committed to safeguarding the moral values that made made this Republic great, including the educational, religious. and parental freedoms of Torah adherents, defending family values, opposing abortion on demand, protecting the moral environment, opposing the radical LGBT (To'aiva) agenda, including opposing legislation of civil unions, as well as defending the security of our brothers internationally, particularly in Eretz Yisroel."