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Sanders, a Republican who has a lesbian daughter, has been a vocal support of the same-sex marriage movement in the last five years.

"Marriage strengthens families, and we need more of that in this country, not less," said Sanders, speaking on camera with the downtown San Diego skyline as a background.

The ad is sponsored by the Human Rights Campaign and a group called Freedom to Marry.

He said former Vice President Dick Cheney and his wife Lynne, former first lady Laura Bush, one-time U.S. Solicitor General Ted Olson and Cindy McCain -- wife of 2008 GOP presidential nominee Sen. John McCain, R-Ariz. -- are stalwarts leading a change in views within the party on the subject.

In January, Sanders attended a news conference with the mayors of other big U.S. cities to express support for gay marriage. He publicly switched positions on the issue in 2007.

He said that he cannot tell his daughter, Lisa, that her relationship with her partner is less meaningful than the one he has with his wife.

The convention begins on Monday in the Florida city, which is under a threat of being hit by Tropical Storm Isaac. The storm is expected to become a hurricane after it enters the Gulf of Mexico.