Catholic Charities provides social services, some of which are city contracts like the management of homeless shelters, to about 68,000 people. The marriage bill would not require churches to perform same-sex weddings, but because Catholic Charities uses city money, the archdiocese fears it would have to offer employee benefits to married same-sex couples.

The marriage legislation is expected to pass next month and has the mayor's support.

Jane G. Belford, the chancellor of the Washington archdiocese, wrote Councilman Phil Mendelson on Wednesday asking for an exemption to protect the church's religious freedoms. The church wants to be exempt from any measure that would require it to extend benefits to same-sex couples.

Council member Tommy Wells said it would be dangerous to let the Catholic Church start writing D.C. laws, a sentiment expressed by Mendelson and other members.

"Allowing individual exemptions opens the door for anyone to discriminate based on assertions of religious principle," Mendelson said. "Let's not forget that during the civil rights era, many claimed separation of the races was ordained by God."

Archdiocese spokeswoman Susan Gibbs said the law would require city contractors to ignore their religious principles.

"We are not threatening to walk out of the city," Gibbs said. "The city is the one saying, 'If you want to continue partnering with the city, then you cannot follow your faith teachings.' "

Catholic Charities, one of dozens of nonprofits that partner with D.C. government, manages city-owned shelters that serve about one-third of Washington's homeless population.

Council chairman Vincent Gray said Thursday that the city would have to find another group to provide social services if the church backs out. He said he didn't see any room for compromise.

Council member Jim Graham said the church hasn't abandoned social services in New Hampshire, Connecticut or Vermont after those states began recognizing same-sex marriages.

In Boston, however, Catholic Charities halted its adoption programs in 2006 because Massachusetts banned discrimination against same-sex couples who want to adopt children.