Bolstered by out-of-state money and volunteers, both sides jockeyed Monday to boost turnout for Maine's referendum on same-sex marriage - a contest that could give gay-rights activists in the U.S. their first such victory at the ballot box.

The state's voters will decide Tuesday whether to repeal a gay-marriage bill signed into law in May by Democratic Gov. John Baldacci.

The contest is considered too close to call, and both campaigns worked vigorously - with rallies, phone calls, e-mails and ads - to be sure their supporters cast votes in the off-year election.

If voters uphold the law, it will be the first time the electorate in any state has endorsed marital rights for same-sex couples, energizing activists nationwide and deflating a long-standing conservative argument that gay marriage lacks popular support.

Conversely, a repeal - in New England, the corner of the country most receptive to same-sex marriage - would be a jolting setback for the gay-rights movement and mark the first time voters overturned a gay-marriage law enacted by a legislature. When Californians voters rejected gay marriage a year ago, it was in response to a court ruling, not legislation.

Five other states have legalized same-sex marriage - Iowa, Connecticut, Massachusetts, Vermont and New Hampshire. But all did so via legislation or court rulings, not through a popular vote. By contrast, constitutional amendments banning gay marriage have been approved in all 30 states where they have reached the ballot.