SAN FRANCISCO (AP) – When the U.S. Supreme Court overturned an amendment to the Colorado Constitution that outlawed discrimination protections for gay people, same-sex couples could not enter into civil unions or domestic partnerships anywhere in the nation, much less get married.

But as they seek to persuade a federal judge to strike down California’s ban on same-sex marriages, lawyers for two unmarried same-sex couples are using that 13-year-old decision as their road map – one they expect will eventually lead the high court to take up the marriage issue.

In the Colorado case, Romer v. Evans, the Supreme Court majority held that voters’ dislike of gays and the laws that several cities had approved to shield them from bias motivated the state amendment. Such “animus,” it said, was incompatible with the section of the U.S. Constitution that requires the government to treat its citizens equally absent a compelling reason to do otherwise.

The attorneys behind the challenge to California’s Proposition 8 plan to argue during a pretrial hearing Thursday that by stripping gays of the right to wed, the voter-approved ban runs afoul of America’s founding framework in the same way – and for the same reason.

“Romer is a strikingly similar situation to what we have here. You had a ballot initiative, a majority vote of the people, taking away a right,” said Theodore J. Boutrous Jr., a member of the legal team led by former U.S. Solicitor General Theodore Olson and veteran trial lawyer David Boies. “And there was no justification or rationale other than disapproval by that majority of that group.”

U.S. District Chief Judge Vaughn R. Walker on Tuesday issued a tentative order to fast-track the case in his San Francisco court.

Among the questions he said he wants covered at trial are whether sexual orientation is unchangeable, if permitting same-sex marriage “destabilizes” traditional unions and whether Proposition 8’s ballot history demonstrates the measure had “discriminatory intent.”

California Attorney General Jerry Brown, a defendant in the case, has sided with gay rights advocates and declined to defend the ban, which overturned a California Supreme Court ruling that had legalized same-sex marriages. The state Supreme Court five weeks ago upheld the measure, saying it represented a valid exercise of voters’ authority to amend the California Constitution.

Proposition 8’s sponsors, a coalition of religious conservative groups called Protect Marriage, has been given permission to intervene in the federal case. In court papers, the group’s lawyers rejected the assertions that anti-gay attitudes fueled the November measure and that the 1996 Colorado case was applicable.

“Nothing in California law, either Proposition 8 or otherwise, indicates that Californians harbor animus towards gay and lesbian individuals,” they wrote.

Since the U.S. Supreme Court’s 6-3 decision, attorneys for gay rights and Christian conservative groups have debated whether the Romer decision could be used to expand gay rights. The ruling marked the first time the Supreme Court determined that the Constitution’s equal rights guarantees extended to gays and lesbians.

“The basic point of Romer is that government cannot ever act out of hostility toward a group of people, and whether that is in the context of marriage or anti-discrimination law, the point carries over,” said Suzanne Goldberg, who worked on the case and now directs Columbia Law School’s Sexuality and Gender Law Program.

The ruling has been cited, though so far unsuccessfully, in past challenges to same-sex marriage bans in Nebraska and Florida. At the same time, gay rights groups mostly have shied away from pursuing federal marriage cases in favor of pursuing marriage rights in state courts.

Legal observers on both sides of the debate agree, however, that California’s Proposition 8 presents novel questions

that could make the issue ripe for federal action.

None of the 29 other states with constitutional prohibitions against same-sex marriages, for instance, approved them after same-sex unions already were sanctioned. When the California initiative passed with 52 percent of the vote, an estimated 18,000 same-sex couples had become married, leaving gays in two groups – those who had the right to wed, and those who no longer did.

Patrick Borchers, a law professor at Creighton University School of Law in Nebraska, said Proposition 8’s supporters can offer plausible reasons besides moral disapproval of gays for the marriage ban. They can also try to convince Judge Walker, the 9th U.S. Circuit Court of Appeals and finally the Supreme Court that states should be allowed to work out their marriage laws on their own.

But if the courts apply Romer as their guiding precedent, he said it is just as likely they would opt to invalidate the California law and recognize gays as a minority group deserving of the kind of protections Colorado’s stricken amendment was designed to curb at the state level.

“You can certainly make the argument that’s being made by Boies and Olson with a completely straight face,” he said. “But if the Supreme Court were to ultimately buy that, then I think they would have to come out and say that gays and lesbians are a protected class.”

The case is Perry v. Schwarzenegger, C 09-2292 VRW.