The McCains' daughter Meghan, who has been outspoken in her support for gay rights, has also endorsed NOH8. Posted on her Twitter site is a photo where she appears with silver duct tape across her mouth and "NOH8" on a cheek.
The Web site said Cindy McCain contacted NOH8 and offered to pose for the photo endorsement.
John McCain's office said in a statement that the Arizona senator respects the views of members of his family but remains opposed to gay marriage.
"Sen. McCain believes the sanctity of marriage is only defined as between one man and one woman," the statement said.
An Arizona ballot measure, passed by voters in 2008 that defined marriage as between one man and one woman, was backed by John McCain.
On Wednesday, in a Twitter message linked to her blog, Meghan McCain said: "I couldn't be more proud of my mother for posing for the NOH8 campaign. I think more Republicans need to start taking a stand for equality."
Wednesday, McCain's Senate re-election campaign said that his presidential running mate, former Alaska Gov. Sarah Palin, will come to Arizona to campaign for him in March. Palin has been a vocal opponent of gay marriage.
