David A. Catania said he will make his break from the Republican Party official today, ending months of public disagreement with national party leaders over their support for an amendment to the U.S. Constitution that would ban same-sex marriage.

Catania, 36, who joined the Republican Party when he was 16, said in an interview yesterday that he will change his registration to independent in city voting records. For the past seven years, he has been an at-large member on the D.C. Council, and his change in party affiliation will leave only one Republican on the 13-member body.

"On a personal level, this is extremely painful and difficult, because for many years the members of this party have been like family to me, especially here in the District," he said. "It's a difficult choice but one I feel I have no choice but to make. I will no longer rationalize my association with a political party that has so badly betrayed my values and principles."

The rift began in January when President Bush hinted at his support of a federal ban on same-sex marriage during his State of the Union address. At the time, Catania, who is openly gay, was an enthusiastic backer of Bush. He had raised nearly $80,000 for the president's reelection and was touted on the campaign's Web site as a "Maverick" -- a top fundraiser under the age of 40.

But when Bush in February formally called for a constitutional amendment, Catania, saying the party was seeking to "write discrimination into the U.S. Constitution," asked that his name be removed from the campaign's literature and publicly called for Bush's defeat in November.

In June, local GOP leaders stripped Catania of his spot in the D.C. delegation to the Republican National Convention in New York, prompting him to resign from the local organization. On the eve of the convention, Catania endorsed Democrat John F. Kerry for president. Kerry has said that although he does not support same-sex marriage, he is opposed to a constitutional ban.

Catania said: "I think that John Kerry has the temperament, demeanor and experience to be the best president. Do I believe him to be perfect? Absolutely not. But do I think the risks associated with another Bush administration profound? You bet."

He said he did not join the Democratic Party because "I felt like I've been stung by one party. I'm not eager to join another. But mostly, being an independent just suits me."

The Log Cabin Republicans, a group of gay activists, declined to endorse Bush for reelection, in large part because of the marriage amendment, but its leaders have insisted that gay members are committed to staying and fighting within the party.

Catania said he has "fought harder than anybody else on behalf of an urban agenda in this party. . . . The federal marriage amendment was certainly the straw that broke this camel's back."

Carol Schwartz, the other Republican at-large member on the council, boycotted the national convention after Catania was booted out of the delegation. "David was an asset to the party both locally and nationally. It's a loss," she said.

Betsy Werronen, chairman of the D.C. Republican Party, also lamented Catania's departure.

"I'm very, very sad. We went through a lot of trauma over this . . . but he's got to do what he thinks is right for him," said Werronen, who has said she refused to certify Catania as a delegate because of his refusal to support Bush for reelection.

"He's been a close friend of ours, and I wish him well. He's a brilliant legislator, and I hope that he continues legislating," she said.

Werronen said she was disappointed that her party will lose a seat on the council but said she will continue trying to recruit candidates for elected office.

As an independent, Catania will not have to relinquish his seat on the council; the city's charter mandates that at least two of the four at-large council seats be set aside for non-majority party candidates. Catania, who stunned the city's political establishment by winning the seat in a special election in 1997, is in the middle of his second four-year term. He said he has not decided whether to seek reelection in 2006.

D.C. Council member David A. Catania said he is leaving the Republican Party because of national support for a ban on same-sex marriage.