But he predicted that Obama's endorsement would change relatively few votes on the ballot measure. Instead, he said the president's announcement may prompt a backlash, particularly among anti-Obama voters who feel "intimidated" by proponents of gay marriage.
In the past, greater intensity among same-sex marriage opponents has given them outsized strength at the ballot box. Wolfson, of Freedom to Marry, acknowledged that gaining a majority among the electorate as a whole is "an uphill fight for any minority group," which is why minority rights are not usually won by referendum but by court or legislative action.
Opposition to gay unions from African Americans has also bolstered proponents of traditional marriage. In 2008, more than 9 in 10 black voters in California backed Obama, then overwhelmingly voted for Proposition 8, the successful ballot measure to overturn the state Supreme Court's decision allowing same-sex marriage.
Derek McCoy, of the Maryland Marriage Alliance, which is attempting to invalidate that state's gay marriage law, predicted Obama's endorsement would sway few African Americans there, where they make up about one-fourth of the electorate.