"It reinvigorates our campaign even more," said McCoy, pastor of a Prince George's County church. He said he has heard from black voters who are disgusted and will now vote against Obama in addition to the marriage law.
The alliance says it has gathered about 45,000 of the 55,736 signatures needed to put the gay-marriage measure on the ballot. It has seven more weeks to meet the target and expects to exceed it by tens of thousands.
Rachael Rice, a top Democratic fundraiser in Maryland who supports the gay-marriage law, agreed that presidential support could have the unintended consequence of motivating opponents. "It could be the last straw," she said.
The thinking, Rice said, could go like this: "The president is for it? We really need to join forces. It is really coming from the top down."
Democratic strategists and gay-marriage proponents hope that the way Obama made his announcement — discussing factors that contributed to his "evolution," including conversations with his family and gay staff members — will provide a road map for undecided or wavering Marylanders to do the same.
"I think he made it OK to change your mind about marriage," said Martha McKenna, a political strategist with clients in Maryland and nationally. Over the next six months, "people who haven't given marriage a lot of thought will be considering the issue," she said.
Another possible impact is more subtle: In 2008, opponents defeated a gay-marriage initiative in California in part by highlighting Obama's former position and reminding voters that he favored civil unions and opposed same-sex marriage.
On MSNBC's "Rachel Maddow Show" on Wednesday night, Massachusetts Democrat Barney Frank — the first openly gay member of Congress — said opponents will be unable to use Obama's words against same-sex marriage in Maryland.
McCoy played down the importance of losing Obama as an unwitting ally. After all, he noted, Obama sided with gay advocates during the North Carolina referendum — but without much success.
Baltimore Sun reporter Michael Dresser and the Los Angeles Times contributed to this article.