Same-sex marriages could begin as soon as tomorrow in the US state of Oregon, depending on a judge's ruling.

US district judge Michael McShane said he'll issue his ruling at midday local time (7am tomorrow NZT) on a constitutional challenge to the state's gay-marriage ban. Officials in Oregon's largest county, Multnomah, say they'll begin issuing marriage licenses immediately if his ruling allows it.

Oregon law has long prohibited same-sex marriage, and voters added the ban to the state constitution in 2004. Four gay and lesbian couples brought the Oregon cases, arguing the marriage laws unconstitutionally discriminate against them and exclude them from a fundamental right to marriage.

McShane hasn't signaled how he'll rule, but the state refused to mount a defense of the voter-approved ban, and both sides asked that it be found unconstitutional.

In refusing to defend the ban, Democratic attorney general Ellen Rosenblum said there were no legal arguments that could support it in light of decisions last year by the US Supreme Court. She sided with the couples, asking the judge to overturn the ban.

The US Supreme Court last year struck down the 1996 Defense of Marriage Act, which barred the federal government from recognizing same-sex marriage. It determined the law improperly deprived gay couples of due process.

Federal or state judges in the states of Idaho, Oklahoma, Virginia, Michigan, Texas, Utah and Arkansas recently have found state same-sex marriage bans to be unconstitutional. Judges also have ordered Kentucky, Ohio and Tennessee to recognize same-sex marriages from other states.

But opposition remains stiff in many places. Critics note most states still do not allow gay marriage and that in most of those that do, it was the work of courts or legislatures, not the will of the people.

Gay rights groups previously said they've collected enough signatures to force a statewide vote on gay marriage in Oregon in November. But they said they would discard the signatures and drop their campaign if the court rules in their favor by May 23.