PORTLAND, Ore. -- Oregon's attorney general will not defend the state's ban on gay marriage, joining fellow Democrat attorneys general in at least five other U.S. states who have made similar decisions.

In documents filed in federal court on Thursday, Ellen Rosenblum argued Oregon's ban cannot withstand a federal constitutional challenge. Her decision comes less than one month after a federal judge decided to consolidate two lawsuits alleging the West Coast state's ban violates the U.S. Constitution.

Attorneys filed the first lawsuit in October, on behalf of two women who have been in a relationship for more than 30 years and another couple who sought to have their union recognized in Oregon. The American Civil Liberties Union then filed a lawsuit two months later on behalf of two same-sex couples.

The first suit argued that limiting marriage to opposite-sex couples violates the due process and equal protection rights of same-sex couples.

Nationally, attorneys general in five states -- Virginia, Pennsylvania, California, Illinois and Nevada -- have declined to defend same-sex marriage bans against lawsuits filed by gay couples, while a sixth, in New Mexico, challenged longstanding legal interpretations that said such unions were impermissible there.

The legal landscape in the U.S. for gay marriage has rapidly shifted in recent years. In a movement that began in 2004, 17 states plus the Washington capital district now allow gay marriage, most of them clustered in the Northeast.

Momentum especially surged after the U.S. Supreme Court ruled last year to strike down part of a federal law that prevented the government from recognizing same-sex marriages. And recently, an appeals court found that gays and lesbians cannot be precluded from jury duty because of their sexual orientation. That ruling extended civil rights protections to gays that the U.S. Supreme Court previously promised only to women and racial minorities.

The two decisions create an atmosphere in which Rosenblum and other attorneys general believe will make defences of gay-marriage bans unlikely to succeed.

The Democrat running for Colorado attorney general called on the current Republican officeholder to stop defending the state's prohibition. And in Texas, Democratic gubernatorial candidate Wendy Davis demanded that her likely Republican opponent, Attorney General Greg Abbott, do the same.

Gay marriage supporters in Oregon celebrated Rosenblum's statement Thursday, and said the decision could preclude an expensive political fight.

"No one is interested in engaging in an expensive political campaign if we don't have to," said Mike Marshall of Basic Rights Oregon, a group readying a ballot measure to overturn the state's gay marriage ban in November.

But gay marriage opponents, including the National Organization of Marriage, said Rosenblum is shirking her duties as the top lawyer for her state.

"She swore an oath of office that she would enforce all the laws, not just those she personally agrees with," said Brian Brown, the group's president. "Ms. Rosenblum is dead-wrong in her conclusion that the amendment cannot be supported by rational legal arguments."