Gay couples were poised to tie the knot immediately, but a national group opposed to same-sex marriage asked an appeals court to block them.
Officials in Oregon's largest county, Multnomah, say they'll begin issuing marriage licenses immediately if U.S. District Judge Michael McShane's decision allows it.
McShane hasn't signaled how he'll rule, but both sides in the case have asked that the voter-approved ban be found unconstitutional.
The judge last week denied a request by the National Organization for Marriage to defend the law on behalf of its Oregon members. On Monday morning, the group appealed that denial to the 9th U.S. Circuit Court of Appeals, seeking an emergency stay of the decision.
Seventeen states and the District of Columbia allow gay marriage. Federal or state judges in 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 point out that most states still do not allow gay marriage and that in most that do, it was the work of courts or legislatures, not the people.
Four gay and lesbian couples brought the Oregon cases, arguing the state's marriage laws unconstitutionally discriminate against them and exclude them from a fundamental right to marriage.
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 U.S. Supreme Court. She sided with the couples, asking the judge to overturn the ban.
Gay rights groups previously said they've collected enough signatures to force a statewide vote on gay marriage in November. But they said they would discard the signatures and drop their campaign if the court rules in their favor by May 23.
The 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.
Oregon law has long prohibited gay marriage, and voters added the ban to the state constitution in 2004. The decision, approved by 57 percent of voters, came months after Multnomah County briefly issued marriage licenses to same-sex couples.
About 3,000 gay couples were allowed to marry before a judge halted the practice. The Oregon Supreme Court later invalidated the marriages.
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