WASHINGTON - A group of US mayors is set to announce on Friday a largely symbolic push to legalize same sex marriage in states across the country, echoing what they say is growing popular support for marriage equality.

The mayors' announcement will be made at the US Conference of Mayors meeting in Washington. While composed mostly of Democrats, the group of 75 mayors also includes Jerry Sanders, the Republican mayor of San Diego, and New York City Mayor Michael Bloomberg, who is an independent.

In addition to Bloomberg and Sanders, Democrats Rahm Emanuel of Chicago, Thomas Menino of Boston and Antonio Villaraigosa of Los Angeles will serve as the group's co-chairs.

While marriage laws are dictated by states and the federal government, rather than cities, the advocacy group Freedom to Marry, which is coordinating the effort, is hoping the mayors can help sway opinion.  

"Mayors are the elected officials closest to the people, and they know what's going on in their communities and they see that marriage strengthens communities," said Marc Solomon, the National Campaign Director of Freedom to Marry.  

The issue promises to feature prominently in the 2012 elections. President Barack Obama has supported rights for same sex couples, but has stopped short of endorsing marriage, while most of the Republicans seeking to challenge him in November have come out against it.  

Some 30 states explicitly ban gay marriage -- and it is strongly opposed by social conservative groups -- but the number of states that allow it is growing.

Last year, New York became the most populous state to legalize same sex marriage. Five other states allow it: Massachusetts, Connecticut, Vermont, New Hampshire and Iowa. Gay marriage is also legal in the District of Columbia.

New Jersey and Washington state are now considering legislation to legalize gay marriage, and gay rights activists in Maine say they plan to bring the issue to voters in a referendum.

The National Organization for Marriage, which advocates that marriage be defined as being between a man and a woman, has said it will spend $250,000 to help fund primary challenges of Republican lawmakers in Washington who vote in favor of gay marriage, and another $500,000 in New Jersey to support lawmakers who oppose gay marriage.

Still, most polls now show a majority of Americans support gay marriage, with the strongest support coming from young people. Last May, Gallup found for the first time that a majority of Americans believe "same-sex marriage should be recognized by the law as valid, with the same rights as traditional marriages."