New York Times:

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.
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 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.