For the first time in American history, voters in three states are poised to legalize gay marriage via popular vote. So far, only state legislatures and courts have sanctified same-sex marriage–it has failed to pass every time it has appeared on a U.S. ballot. All that could change Tuesday, when voters in Maryland, Washington and Maine are expected to approve gay marriage via referendum.

Gay marriage advocates say the stakes could not be higher. “We think we are at a turning point, and while we believe that civil rights should not be put to a vote, now that we are in this we need to win,” says Brian Ellner, co-founder of The Four 2012, a campaign to support gay marriage in Maryland, Washington, Maine and Minnesota, where a constitutional amendment is on the ballot to define marriage as between one man and one woman. “In order to make our case to the Supreme Court, in order to continue to make our case to public officials and others who support us, we need to prove in at least one of these states that we can win a popular vote.”

Polls show voters in these states favor approving gay marriage, but the margins aren’t huge. In Washington, support for gay marriage leads 58% to 37%, according to an Oct. 31 Washington Poll. Support in Maine, where voters narrowly rejected a same-sex marriage law in 2009, is similarly strong. According to a Critical Insights poll last week, likely voters in Maine support the referendum 55% to 42%, but opposition this time has increased 7% since the same poll was conducted in June. Marylanders narrowly support gay marriage 52% to 43% according to a recent Washington Post poll.

Opponents of gay marriage however say that a loss won’t spell trouble for their movement. “We are being outspent by an average of four-to-one,” says Brian Brown, president of the National Organization for Marriage. “If our opponents were to succeed in winning somewhere, it would be a result of the heavy Democratic majorities in those states that are on the ballot, and their ability to marshal tremendous resources behind their campaigns. It won’t reflect any fundamental shift.” Even if they do not win this time, Brown argues, “Marriage cannot be redefined through political means.”