WASHINGTON — Colorado’s freshly passed law that gives gay couples the right to form a civil union could be dramatically expanded depending on how the U.S. Supreme Court rules on California’s ban on gay marriage.

The high court justices will hear arguments Tuesday and Wednesday on the constitutionality of both California’s Proposition 8, which bans same-sex marriage in that state, and the Defense of Marriage Act, which prohibits the federal government from recognizing gay marriage, even in states that permit it.

Prop 8’s arguments could be big for Colorado.

Since 2006, Colorado’s gay rights advocates have been hamstrung by a constitutional state ban on gay marriage.

So earlier this month, the Democratic-controlled state legislature approved civil unions instead, acknowledging it’s not as good as marriage, but saying the new law grants protections that didn’t exist before Gov. John Hickenlooper signed it into law last week.

But the high court judges could not only declare California’s Prop 8 unconstitutional, they could rule that states, like Colorado and eight others, that give gay people some relationship distinction without calling it “marriage” is unfair.

“It essentially says you have a separate and unequal status,” said Brian Moulton, legal director for the D.C.-based Human Rights Campaign, which advocates for gay rights. “They could rule in a way that someone in Colorado could easily take that court’s opinion and go to court, or to the governor, and say that clearly our situation is very similar … and we want marriage.”

Colorado is one of only seven states that have both a gay relationship distinction — either civil unions or domestic partnerships — and some sort of ban on gay marriage.

That dichotomy is a symbol of the state’s — and nation’s — rapidly changing views on gay marriage.

In matter of a few years, polls show Colorado has shifted to more than half of voters approving of gay marriage, according to Public Policy Poll of 542 Colorado voters in April. (In 2006, 55 percent of Colorado voters supported a ban on gay marriage.)

“It’s really because neighbors and families and friends have come out, and more and more Americans know someone who is gay or lesbian and the more they know them, the more they love them and want them to have the same protections,” said Brad Clark of One Colorado.

Even President Barack Obama, who said in 2008 that he didn’t support gay marriage, announced he was in favor of it before his re-election campaign last year.

This is why, in part, Kris Miccio, a University of Denver law professor and a lesbian, was against civil unions to begin with.

“From a moral perspective, it raises amazing issues for the state of Colorado … for me, the idea in Colorado that we would agree to a second-class citizenship is unbelievable,” she said. “If some people get marriage, all people should get marriage.”

Miccio said she and her partner plan to marry this year in Massachusetts, but when they go back to Denver County, it will be recognized only as a civil union.

Clark argued the patchwork of relationship distinctions and bans “creates a really burdensome system especially on national employers who are trying to deal with the definition of same-sex couples in different ways.”

Clark said a relationship status could literally change on a commute to work as people drive through different counties.

While most gay marriage advocates hope for the sweeping Supreme Court victory — a ruling that broadly declares all state bans unconstitutional and recognizes other relationship distinctions as marriage — most say it is unlikely.

Justices also could decide on California’s law narrowly and let states go from there, fighting one by one.

In Colorado, that could lead to a ballot initiative, Clark said. “But a lot of it depends on the language the court uses.”

Allison Sherry: 202-662-8907, asherry@denverpost.com or twitter.com/allisonsherry