A federal judge has struck down Michigan's ban on gay marriage, the latest in a series of decisions overturning similar laws across the U.S.
That means clerks could start issuing licenses Monday unless a higher court intervenes.
Judge Bernard Friedman ruled Friday after a trial that mostly focused on the impact of same-sex parenting on children.
Two nurses who've been partners for eight years claimed the ban violated their rights under the U.S. Constitution.
Michigan Attorney General Bill Schuette said he was immediately filing a request with a federal appeals court to suspend Friedman's decision and prevent same-sex couples from immediately marrying.
Seventeen states and the District of Columbia issue licenses for same-sex marriage. Since December, bans on gay marriage have been overturned in Texas, Utah, Oklahoma and Virginia, but appeals have put those cases on hold.
Two nurses, Jayne Rowse and April DeBoer, want to get married, but the original purpose of their 2012 lawsuit was to overturn Michigan's ban on joint adoptions by same-sex couples.
They are raising three adopted children with special needs, but they can't jointly adopt each other's kids because joint adoption in the state is tied exclusively to marriage.
Rowse, 49, and DeBoer, 42, didn't testify, and the trial had nothing to do with their relationship. In fact, attorneys for the state told the judge that they are great parents.
Instead, the state urged the judge to respect the results of a 2004 election in which 59 per cent of voters said marriage in Michigan can only be between a man and a woman. Conservative scholars also questioned the impact of same-sex parenting on children.
But experts testifying for Rowse and DeBoer said there were no differences between the kids of same-sex couples and the children raised by a man and woman.
The couple got a standing ovation and cheers Friday at Affirmations, a community centre for gays and lesbians. DeBoer said she and Rowse won't get married until the case comes to a close in the months — possibly years — ahead.