Mayors Rahm Emanuel of Chicago, Mike Bloomberg of New York City and Jerry Sanders of San Diego are among 70 mayors pledging to support gay marriage.

The mayors will announce the launch of Mayors for the Freedom to Marry at a press conference on Friday, January 20 during the U.S. Conference of Mayors, their annual Washington D.C. gathering.

Mayors for the Freedom to Marry is backed by Freedom to Marry, one of the nation's largest groups working to win marriage rights for gay and lesbian couples.

“By joining the group, mayors hope to expand public and political support for ending the exclusion of same-sex couples from marriage,” the group said in a statement.

Mayors for the Freedom to Marry will be co-chaired by Mayors Sanders, Thomas Menino of Boston and Antonio Villaraigosa of Los Angeles.

Sanders, a Republican, provided emotional testimony during the second week of the trial that found California's gay marriage ban, Proposition 8, unconstitutional. He testified that he once opposed marriage equality. In 2007, he was prepared to veto a San Diego City Council-approved resolution calling on the city to file an amicus brief in support of a San Francisco legal challenge. Instead, with his wife by his side, Sanders held a tearful press conference and told reporters: “I have close family members and friends who are members of the gay and lesbian community. Those folks include my daughter Lisa.”

Also involved in the campaign are Mayors Annise Parker of Houston and Marilyn Strickland of Tacoma.