It is halachically forbidden to support the congressional campaign of Democrat David Weprin, a group of Flatbush Orthodox rabbis decreed this week.

A letter signed by the group cites the Queens Assemblyman’s vote in favor of same-sex marriage in July, insisting the vote was a chillul Hashem, or desecration of God’s name.

"Weprin’s claim that he is Orthodox makes the chillul Hashem even greater,” states the letter, which was posted online by the Haredi site Matzav.com. The letter forbids voting for, campaigning for, funding or otherwise supporting the campaign of Weprin, who grew up in a traditional Jewish home, attended yeshivas and chose to become Orthodox following his bar mitzvah 42 years ago.

Among the signatories were Rav Yisroel Belsky, Rav Yisroel P. Gornish, Rav Simcha Bunim Cohen, Rav Shmuel Kamenetsky, Rav Avrohom Yaakov Nelkenbaum, Rav Hershel Zolty, Rav Grainom Lazewnik, Rav Eliyahu Brog, Rav Chaim Benoliel, Rav Max Maslaton, Rav Yaakov Yisroel Rubin, Rav Shlomo Stern, Rav Yosef Eisen, Rav Yitzchok M. Landau, Rav Rahamim Harary, Rav Chaim Krauss, Rav Yosef Leifer, Rav Shmuel Friedler, Rav Mechel Rosenbaum and Rav Pinchos M. Twerky, Matzav reported.

Only a small part of Flatbush is included in the 9th Congressional District, which spans from southern Brooklyn to the Rockaways and into central Queens.

The letter was released as a new poll shows Republican Bob Turner with an edge over Weprin in the congressional race to succeed former Rep. Anthony Weiner.

Turner leads Weprin by a margin of a 50 percent to 44 percent, the Siena College survey found. The poll, conducted September 6-8, showed Weprin, an Orthodox Jew, leading Turner among Jewish voters by 51 percent to 45 percent.

The two are facing off in a Sept. 13 special election in a heavily Jewish New York congressional district spanning parts of Queens and Brooklyn. The race is being closely watched nationally as a bellwether for how the party of the president is faring.

Overall, momentum appears to be shifting in the Rpublican candidate’s favor. A Siena College poll in early August had shown Weprin leading Turner overall by 6 percentage points, with a much wider lead among Jewish voters than he currently enjoys.

On Thursday, the Democratic Congressional Campaign Committee announced it would spend $500,000 to air advertisements supporting Weprin in the Democratic-dominated district, Politico reported.

The Jewish vote has loomed large in media coverage of the race. Weprin, a New York state assemblyman, has been criticized by some within the Orthodox community for his vote to legalize same-sex marriage. Fellow Democratic New York State Assemblyman Dov Hikind, who represents Orthodox neighborhoods in Brooklyn and has blasted Weprin on the issue, endorsed Turner on Wednesday.

Also on Wednesday, The Jewish Press, a prominent Brooklyn-based newspaper serving the Orthodox community, endorsed Weprin, citing his support for social welfare programs and his opponent’s calls for government spending cuts.

Previously, New York City’s former mayor, Ed Koch, a Democrat, had urged voters to support Turner in order to send a rebuke to President Obama for his policies toward Israel. Weprin himself has criticized Obama’s policies toward Israel.

The latest poll found that only 42 percent of the district’s Jewish voters had a favorable opinion of Obama, while 54 percent viewed the president unfavorably.

Sixteen percent of the district’s Jewish voters said that a candidate’s Israel stance would be the most important factor in determining their vote, roughly half the proportion (30 percent) who identified the candidate’s position on the economic recovery as their key issue and slightly fewer than the proportion (20 percent) who chose Social Security, Medicare and other so-called entitlement programs as the top factor.

On Thursday night Turner held an event to get out the Jewish vote outside a kosher supermarket in heavily Orthodox Kew Garens Hills.