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. But the measure is likely to draw harsh criticism from congressional Republicans and conservative Democrats, many of whom face mid-term 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 Barack 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 Massachusetts, Connecticut, Iowa, and Vermont. In May, Washington passed legislation to recognize such unions from other jurisdictions, and Congress did not try to override that decision. Rep. Jason Chaffetz, a Republican from Utah, said he did not believe his fellow opponents of same-sex marriage would be able to block the city's measure legalizing it. 'Given the other issues Congress is focused on, such as health care, it hasn't got much attention,' said 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 piqued race and class tensions, as most of the vocal opponents represent inner-city black churches, while the more liberal and white population largely backs the measure. The City Council has two openly gay members and around 5 percent of the city's couples identify themselves as gay or lesbian, the second highest rate in the country, according to a 2000 survey by the Human Rights campaign. If passed, the city would phase out its local domestic partnership law and instead allow two persons who are currently in a valid domestic partnership, as recognized by the city, to apply for and receive a marriage license free of charge. 