Because Mayor Bloomberg and all the Democratic mayoral candidates support gay marriage, some religious leaders and other opponents of same-sex nuptials have said they must decide between voting for someone who doesn’t share their values and sitting out the election entirely. One mayoral candidate, however, does oppose gay marriage – and may threaten the mayor’s re-election bid from the right because of it, according to some followers of New York conservative politics.

One of Mr. Bloomberg’s two Republican challengers, Thomas Ognibene, is the only mayoral candidate against same-sex marriage. “I think that marriage is at the root of the Judeo-Christian ethic we live under. I’m totally opposed to gay marriage,” Mr. Ognibene, a former member of the City Council from Queens said.

Mr. Ognibene, who is Catholic, said the ruling February 4 by a judge of the state Supreme Court at Manhattan, Doris Ling-Cohan, was especially objectionable because it was an example of “a judge overreaching, introducing herself into another branch” of government. The matter of same-sex marriage, he said, is “typically a legislative decision.” Mr. Ognibene also said he voted against domestic partnership laws when they came before the City Council.

Mr. Ognibene, who has the endorsement of the Queens County Republican Party and who sells his candidacy as an alternative for Republicans who demand conservative governance from their candidates and leaders, said the gay-marriage question was just one of the areas in which he is “distinguishable” from Mr. Bloomberg and the rest of the mayoral candidates. He cited his commitment to reducing taxes, the size of government, and health-care costs as other examples.

“When people say, ‘We don’t have a choice because we don’t have a candidate opposed to gay marriage,’ I’d tell them, ‘People do have a choice. That’s the whole purpose of my candidacy,'” Mr. Ognibene said. He said he will attend a City Hall press conference tomorrow at which the City Action Coalition, a group of ministries claiming to represent “a Christian voting bloc that numbers in the tens of thousands,” intends to announce its formal opposition to Justice Ling-Cohan’s decision.

Mr. Bloomberg’s other Republican challenger, Steven Shaw, is also running as a genuine Republican alternative to Mr. Bloomberg. Mr. Shaw, however, said he thinks the mayor did the right thing in appealing Justice Ling-Cohan’s ruling, but he favors giving legal status to same-sex marriage.

Moreover, unlike many Republicans nationwide who decry the hand of the judiciary in making same-sex marriage legal, Mr. Shaw said courts have an important role to play in the matter.

“The issue with just having it in the legislature is that most people oppose gay marriage, but that doesn’t necessarily mean that it is the correct and fair decision, because way back when, most people were in favor of slavery – but if you put that up to the legislature, you could still have something like slavery,” Mr. Shaw said.

This position may hurt Mr. Shaw in his effort to secure the endorsement of New York’s Conservative Party, which, according to its chairman, Michael Long, he and Mr. Ognibene are both seeking. “Candidates supporting gay marriage is clearly a deal-breaker with the Conservative Party,” Mr. Long said yesterday.

He said same-sex marriage might be a deal-breaker with voters in a Republican primary, too.

“If it gets into the Republican Party primaries, I think it will take its toll,” Mr. Long said, adding that any candidates who opposed gay marriage could capitalize on that position in trying to unseat Mr. Bloomberg.

“The overwhelming majority of registered Republicans believe very strongly in the sanctity of marriage, and they’re not pleased with the mayor’s position,” the party chairman said. In the primaries and in the general election, Mr. Long said, it will be “a very major problem for him.”

The mayor’s camp, however, seemed unconvinced.

“We’re not concerned about Ognibene’s stand. That’s his business,” a spokesman for the mayor, William Cunningham, said yesterday.

“We’ve had a Republican primary before, and if we have one this year we’ll deal with it,” Mr. Cunningham said. He also questioned politicians who maintain that “Republicans won’t support a pro-choice, pro-gun-control, and pro-gay-rights mayor,” because Mayors Giuliani and Bloomberg both fit that description.