The media campaign will cost $250,000; its organizers said it was the largest-ever purchase of print advertising space by gay-rights supporters.

Signatories of the ads included Salt Lake City Mayor Rocky Anderson and the mayors of Boston; Los Angeles; San Francisco; Seattle; Providence, R.I.; Portland, Ore.; West Sacramento, Calif., and Palm Springs, Calif.

Roberta Sklar of the National Gay and Lesbian Task Force said the ads would run in papers including The New York Times and The Gazette of Cedar Rapids, Iowa, as well as gay weeklies in Houston, Atlanta and San Diego.

The ads feature photographs of five same-sex couples who have been together as long as 53 years and are endorsed by an array of organizations and individuals, including 11 religious leaders and nine mayors.

''From coast to coast, millions of people and hundreds of organizations are working to protect gay and lesbian families by ending their exclusion from marriage,'' the ads say. ''Along the way, there will be advances and setbacks, but we will not stop until every American family is treated fairly, with dignity and equality under the law.''

The ad placement comes at an eventful time for the same-sex-marriage debate.

Opponents of same-sex marriage have recently won important court rulings in several states. However, a proposed federal constitutional amendment to ban same-sex marriage failed to win two-thirds support in both the Senate and House.

''There's no question the landscape is mixed,'' said Evan Wolfson of Freedom to Marry, an organizer of the ad campaign along with the task force and the Gay and Lesbian Alliance against Defamation.

''This is a long-term conversation,'' Wolfson said. ''Our job is to make sure people hear about gay families and why marriage matters, and not be drowned out by the horse race of the moment.''