Any effort to amend the constitution requires lawmakers to sign off on the measure twice before sending the issue to the voters. With one vote by the Republican-dominated General Assembly already down, and a second approval a foregone conclusion, Sutton said his group's efforts would shift to the ballot box, where voters could see something "probably on the level of a governor's race."
"I believe we'd see a large-scale effort, not unlike a statewide political campaign," he said, noting his group has had already drafted a campaign plan for 2014.
Supporters of the ban, which could take top billing when lawmakers return in January, called the report a scare tactic designed to spook lawmakers who already voted overwhelmingly to approve the ban in 2011.
Micah Clark, executive director of the American Family Association of Indiana, said supporters of the ban modeled the Indiana measure off of ones approved a decade earlier in Wisconsin and Kentucky.
"It's been in place for 10 years with no negative consequences. It has not affected the universities. It has not affected the businesses. It's simply kept marriage as defined in Kentucky and Wisconsin law, and we think it would do the same thing here," he said.
Republican House Speaker Brian Bosma has said the ban could get quick approval from lawmakers if it comes up, but he did not include it in an official packet of proposals Republican leaders say they will focus on. Opponents of the ban, including Sutton's group, said they would do everything they can to keep the issue in front of lawmakers.
Maryland, Maine and Washington were the first states to pass same-sex marriage by popular vote. Six other states — New York, Connecticut, Iowa, Massachusetts, New Hampshire, Vermont — and the District of Columbia already allowed gay marriage. But the other states' laws were enacted either by lawmakers or court rulings.
"I believe the national tide has shifted dramatically since" lawmakers first voted on the ban in 2011," Sutton said. "I think we've not slipped backward, but moved forward."
Supporters of the ban say the new report is part of an effort to drag out a debate that was essentially already decided by lawmakers last year, and already written in the state law.
"The issue is really easy to understand for Hoosiers and for lawmakers: Should marriage between one man and one woman be protected?" said Eric Miller, president of Advance America, an Indiana-based group supporting the ban. "What we're doing with a constitutional amendment is not changing the law; we're protecting the law from being changed by a judge or court."