It’s a Rosh to stop Mamdani.

A Brooklyn synagogue isn’t letting congregants score High Holidays seats if they aren’t registered to vote — a drastic step amid Sephardic Jewish congregations’ fear over the potential election of Israel critic Zohran Mamdani as mayor.

A notice from Congregation Shaare Zion in Gravesend obtained by The Post warns that the election’s outcome could pose “serious problems” throughout the city and left the synagogue “no choice” but to require proof of voter registration for tickets to Rosh Hashana and Yom Kippur services.

Mamdani isn’t directly mentioned in the announcement, but a source with the Orthodox synagogue said the move was directed at the Democratic mayoral nominee.

“It’s an effort to defeat Mamdani and to make sure the Jewish community’s voice is heard on the most important day in the electoral calendar, Election Day,” a synagogue source said.

“They were alarmed that someone who has expressed antisemitic sentiment in the past such as the phrase ‘globalize the intifada’ and supporting the boycott against Israel.”

Mamdani, who would be New York City’s first Muslim mayor if elected, has repeatedly contended he’s not antisemitic. He has supported the boycott, divest and sanctions movement against Israel, pledged to arrest Israeli Prime Minister Benjamin Netanyahu if he sets foot in New York City and refused to denounce the “globalize the intifada” rallying cry.

Against this background, a coalition of Sephardic rabbis in the Syrian Jewish community recently issued a declaration urging their congregants to vote in November’s election.

“We dare not be passive when our way of life is on the line,” the declaration reads.

“This is not optional. It is a mitzvah,” it adds, using the Hebrew word for good deed.

The Congregation Shaare Zion notice, first reported on by Yeshiva World News, echoes the rabbis’ dire language, warning of a “serious risk” to congregants’ safety, depending on the election’s outcome.

It then goes a step farther by threatening to withhold coveted seats to the High Holidays — a crowded, often-first-come-first-serve time of year for synagogues.

“As a result, we have no choice but to require that all of our members and their spouses provide proof of voter registration prior to securing their High Holiday seats,” the notice states.

A representative at Congregation Shaare Zion declined to comment Friday.

Rosh Hashana will begin at sundown on Sept. 22, and Yom Kippur at sundown on Oct. 1.

A synagogue source said the Sephardic community, which has a stronghold in southern Brooklyn, had largely backed former Gov. Andrew Cuomo in the Democratic primary.

“There’s an expectation they will do so in the general election,” the source said.

Polls show that Mamdani does draw support from a large swath of Jewish New Yorkers, despite widespread misgivings.

The Queens assemblyman had a 17-point lead among Jewish voters, with 43% of their support, according to a Zenith Research and Public Progress Solutions poll from late July.

The poll noted, however, that this support was mainly driven by younger Jewish voters, with Mamdani taking 67% of Jewish Gen Z and millennial ballot casters.

Another recent poll conducted by GQR in partnership with the pro-Israel, Jewish advocacy organization New York Solidarity Network determined that of Jewish New Yorkers surveyed, 37% supported Mamdani’s candidacy.

Mamdani has also garnered support from some Jewish pols, including city Comptroller Brad Lander and Rep. Jerry Nadler.