Not all of the Republicans in town for the party convention this week are celebrating President Bush.

The Log Cabin Republicans, a coalition of gays and lesbians within the GOP, are threatening to withhold their endorsement from Mr. Bush over his support for a constitutional amendment to ban gay marriage.

They are also fuming over a proposed platform plank that opposes both gay marriage and civil unions.

The group’s pre-convention reception yesterday, hosted by Mayor Bloomberg at the Bryant Park Cafe, celebrated the high-profile Republicans officials who endorse their cause, including Mayor Giuliani and Governor Schwarzenegger. Others attend were Governor Pataki, Senator Specter of Pennsylvania, and a former governor of Massachusetts, William Weld.

The event exposed a fracture in the Republican Party as delegates gathered for a four-day show of unity at Madison Square Garden.

Taking their name from Abraham Lincoln’s childhood home, the 26-yearold organization says it represents the best tradition of the Republican Party, which was founded by opponents of slavery. Its members call themselves loyal Republicans who believe in a muscular foreign policy and limited government. They backed Mr. Bush in 2000, but began rethinking that support when he called for a federal marriage amendment earlier this year.

“We’re not going to remain silent while the radical right tries to hijack this party and hijack this presidential election,” said the executive director of the Log Cabin Republicans, Patrick Guerriero.

The group is to announce this year’s endorsement shortly after the convention, raising the possibility of a gesture by the Bush campaign that would lead to rapprochement. A national board member, Patrick Murphy, emphasized the group will either endorse Mr. Bush or no one and will not consider Senator Kerry or any other non-Republican.

“We’re staying in this party to fight for the heart and soul of this party,” Mr. Murphy said.

Mr. Guerriero said he takes comfort in the fact that many of the Republicans who received prime-time speaking slots at the convention – including Messrs. Giuliani, Schwarzenegger, and Pataki – support gay rights. “We think the future of the party is represented by the people in prime time,” he said. He added, however, that the party platform undermines its message of moderation. “You can’t put lipstick on a pig and not call it a pig.”

In his remarks at the reception, Mr. Bloomberg, a supporter of Mr. Bush, made it plain that he disagrees with him on this issue.

“I don’t think we should ever use the Constitution to drive a wedge between us,” the mayor said. “Quite the contrary – the Constitution is there to pull us together.”

Mr. Specter called his support for gay-rights legislation a matter of principle, not politics. “It’s a civil rights issue, and you ought not to count votes on it,” he said. “In the long sweep of history – maybe the short sweep of history – those who support gay rights are going to be on the right side of the issue.”

Outside, a handful of protesters from Gays Against Bush faulted Mr. Bloomberg for what they called “antigay” actions, including his reluctance to endorse gay marriage publicly, his vetoes of certain legislation, his participation in the St. Patrick’s Day Parade that excludes gay groups, and his continued membership in the Boy Scouts.

They also expressed skepticism about the mission of the Log Cabin Republicans.

“I applaud their persistence in trying to get a voice in the Republican Party, but they don’t have a voice,” one protester, Gilbert Baker of Harlem, said of the Log Cabin Republicans. “As swank-o as this party is here, the people are all second-class citizens in the Republican bit tent. They’re not even in the big tent.”