WASHINGTON — City Council members introduced legislation Tuesday to allow same-sex marriage here. If it passes, as expected, Washington would be the first city below the Mason-Dixon line to allow such unions. The city’s bill is expected to become law by December.

David Catania, a Washington councilman, backs an effort to allow gay couples to marry

But the measure is likely to draw harsh criticism from Congressional Republicans and conservative Democrats, many of whom face midterm elections next year, and they could act to overturn it.

After Mayor Adrian M. Fenty, who supports the measure, signs it, Congress has 30 days to enact a joint resolution of disapproval. President Obama would have to sign that resolution for the city law to be blocked.

But even if, as most gay rights advocates predict, such a resolution is not passed, members of Congress could still try to attach a rider to another piece of legislation blocking same-sex marriage here.

“Opposition by some in the House already has been announced,” said Eleanor Holmes Norton, the city’s delegate to the House, adding that she did not believe the opposition would be enough to block the city’s measure.

“Opposition to civil rights is not new,” she said. “We should approach the rights of gay couples and families with the same resolution and results as we had for others who have sought their human rights in Congress and in the District.”

Same-sex marriage is legal in Connecticut, Iowa, Massachusetts and Vermont.

In May, Washington passed legislation to recognize such unions from other jurisdictions, and Congress did not try to override that decision.

Representative Jason Chaffetz, Republican of Utah, said he did not believe his fellow opponents of same-sex marriage would be able to block the city’s measure.

“Given the other issues Congress is focused on, such as health care, it hasn’t got much attention,” said Mr. Chaffetz, the ranking member of the House subcommittee that oversees the District. “You couple that with the Democrats’ stranglehold on House rules, and the minority is left out of the legislative process.”

For the city, the issue has stirred race and class tensions, as most of the vocal opponents represent black churches, while the more liberal and white population largely backs the measure.

Advocates of the bill hope its success will accelerate efforts to pass similar legislation in Maryland.

Maine voters will consider the issue on a ballot initiative in November.

New Hampshire is scheduled to begin allowing same-sex marriages in 2010.

The City Council has two openly gay members, and around 5 percent of the city’s couples identify themselves as gay, the second highest rate in the country, according to a 2000 survey by the Human Rights Campaign.

If the measure passes, the city would phase out its local domestic partnership law and instead allow two people who are currently in a valid domestic partnership to apply for and receive a marriage license free of charge.

But the measure, whose main author was Councilman David Catania, still faces obstacles.

Last month, opponents of same-sex marriage filed a petition for a referendum on the subject. If approved by the Board of Elections and Ethics, the initiative would give city residents the chance to vote next year on whether to legalize same-sex marriage.

“ ‘Let the people vote’ is the cry that is rising among the many ministers and churches in the D.C. area,” said Bishop Harry Jackson, senior pastor of Hope Christian Church and chairman of a group called Stand4MarriageDC.

Bishop Jackson, who helped file the petition for a referendum, said: “The faith community has been concerned for months that it’s been cast as bigots, racists, and worse. Nothing could be further from the truth.”

The board rejected a similar effort in May seeking a referendum on the city’s law recognizing same-sex marriages from other jurisdictions.

Archbishop Donald W. Wuerl has also opposed the legislation.

“Marriage is a path toward holiness,” he wrote in a letter to about 300 Roman Catholic priests. “As members of the church, we are obliged to be all the more attentive to the challenges that weaken marriage.”

Mr. Catania pointed out that the measure protected clergy members from being forced to take part in same-sex marriage ceremonies.

www.nytimes.com/2009/10/07/us/07marriage.html