“What happens in Indiana is critical,” said Brian Brown, president of the National Organization for Marriage, which opposes same-sex marriage. He and other opponents hope the outcome here will reveal that shifts in public sentiment over the last few years are not as widespread as some may think.

Supporters of same-sex marriage, however, are pouring money and effort into defeating the measure in Indiana, a possibility that seemed unthinkable not long ago but one that advocates now insist is conceivable. They say victory in a conservative place like Indiana would be a turning point in a fight that has largely been waged in more predictable, left-leaning states or in the courts. “That would send a clear message to opponents of marriage equality that it’s time to be done fighting this battle,” said Sarah Warbelow, state legislative director of the Human Rights Campaign.

As lawmakers prepare to return for a new legislative session in January, it is an especially awkward spot for Republicans, who dominate both chambers of the General Assembly. With an election year ahead and the risk of primaries in May, the issue is pitting socially conservative groups, who are urging a constitutional ban, against sometime allies in the state’s business community, who say a ban could cause Indiana economic harm.

Few Republicans now seem eager to talk about the issue, and some legislative aides said it was not entirely certain who would formally file the legislation in January.