The U.S. Conference of Mayors (search) will vote Monday on a resolution that opposes a federal constitutional amendment banning gay marriage.

The resolution was unanimously approved Saturday by the Criminal and Social Justice Committee (search), putting it on Monday's agenda for consideration by the larger conference, said conference spokeswoman Rhonda Spears.

The proposed federal marriage amendment would define marriage as between a man and a woman, invalidating any gay marriages nationwide, including those in Massachusetts, which has allowed same-sex marriages (search) since last month.

The mayors' resolution states that the amendment would restrict the ability of local governments to determine marriage policy, and may be interpreted as prohibiting local governments from granting domestic partner benefits.

The resolution was co-sponsored by Boston Mayor Thomas Menino (search), San Francisco Mayor Gavin Newsom (search) and Chicago Mayor Richard Daley (search), who have been strong supporters of gay rights.