Maryland's Board of Elections approved the groups' proposed petition form Wednesday, the first step before they can start collecting signatures.
Gay rights advocates acknowledge that they haven't had success nationally at the ballot box. But they say the dynamic is changing.
In the past, same-sex marriage became law in some states through judicial rulings when marriage laws were challenged in court. Passage of such laws by legislative bodies signals a broader acceptance of same-sex marriage, Rouse said. "It is a new day," he said. "The ground has shifted."
The Human Rights Campaign has already spent $500,000 in Maryland doing outreach in targeted House of Delegates districts. That campaign — and the Maryland General Assembly debate — forced families to talk and think about the issue, he said.
"Let's be frank, those conversations have been uncomfortable," Rouse said. "People's minds and hearts have been opened and changed."
Those who gathered Thursday weren't talking about the referendum just yet. "We want to savor this," said Robyn Zeiger, 60, who attended with her wife, Stacey Williams, 47.
The pair married in a small ceremony in Washington, D.C., but want to have a second, bigger wedding in Maryland, if possible. "I don't want to feel like I have to hide who I am," Williams said. She'd like to have another ceremony where she can "scream down the aisle and jump up and down like everyone else."
The bill has further significance, whether or not it survives referendum. Passage represents a legislative victory for O'Malley, who took up the cause after it was shelved in the House of Delegates last year for lack of votes.
The governor and his staff dedicated considerable time to work with wavering lawmakers to find the 71 votes needed for passage. In a dramatic week during which four delegates switched their positions, the bill squeaked through the body with a single vote to spare.
"This is another piece of what is a growing national profile for Governor O'Malley," said Anita Dunn, an adviser to Obama during his 2008 campaign and his communications director in 2009.
"It is has been a long time since Maryland has had a governor who is seen as a national leader," Dunn said.