Proponents of California’s ban on same-sex marriage asked the US Supreme Court on Tuesday to reverse a lower court decision striking down the state’s 2008 constitutional amendment that had defined marriage as a union between a man and a woman.

In a 39-page petition, Washington lawyer Charles Cooper urged the high court to take up the case and rule that state voters retain the ability to decide for themselves how to define marriage without violating federal constitutional rights.

“While our Constitution does not mandate the traditional definition of marriage, neither does our Constitution condemn it,” Mr. Cooper wrote.

“Rather,” he said, “it leaves the definition of marriage in the hands of the People, to be resolved through the democratic process in each State.”

The California appeal arrives at the high court at the same time as several cases challenging the federal Defense of Marriage Act, which limits federal benefits to married opposite-sex couples.

Should the high court agree to hear the Prop. 8 and DOMA cases, the combination of both issues during the same term would mark the most extensive and important Supreme Court examination of gay rights in the US ever.

It also comes as President Obama and the Democratic Party are considering embracing gay rights as a key priority on the party’s platform during the coming presidential election.

At issue in Hollingsworth v. Perry is whether a panel of the Ninth US Circuit Court of Appeals in San Francisco was correct when it ruled 2-to-1 in February that the 2008 California ballot initiative, Proposition 8, violated the equal protection rights of gay and lesbian couples seeking to marry.

The issue of same-sex marriage arose after the California Supreme Court interpreted the state constitution as requiring the recognition of same-sex marriages.

Supporters of the traditional definition of marriage objected and organized a ballot initiative to amend the state constitution. The Prop. 8 amendment read: “Only marriage between a man and a woman is valid or recognized in California.”

In the months before the statewide vote, 18,000 same-sex couples took advantage of the ruling and were married.

The campaign for and against Prop. 8 was bitter and divisive. Prop. 8 passed with 52 percent of the vote.

After the amendment passed, two same-sex couples filed suit in federal court in San Francisco, charging that Prop. 8 violated the US Constitution. A federal judge conducted a trial and ruled for the couples, declaring a constitutional right to same-sex marriage.

A federal appeals court also ruled for the couples, but on different grounds. The appeals court, in a 2-to-1 decision, said that because California had earlier recognized a right for same-sex couples to marry, it could not later seek to terminate that right in a way that punished same-sex couples.

The appeals court said Prop. 8 was not a reflection of “ill will on the part of the voters of California” against gay and lesbian couples. But the judges condemned the Prop. 8 organizers as having been motivated by a desire to “dishonor,” and to “disapprove of gays and lesbians as a class.”

The Constitution requires equal treatment, including for minority groups subject to discrimination, the US appeals court said.

The decision is controversial because it applies an earlier Supreme Court ruling involving gay rights in Colorado to a different set of facts in California involving gay marriage.

In the Colorado case, the US Supreme Court invalidated a Colorado constitutional amendment that barred local cities and towns from enacting ordinances protecting gay and lesbian residents from discrimination. In striking down the Colorado amendment, the high court said gays and lesbians were entitled to the full protections of the law.

In the California case, the preexisting right to same-sex marriage had been created by a court ruling interpreting the state constitution several months before the Prop. 8 vote unambiguously amended it to say something else.

Cooper argues that states can provide their citizens more rights than are required under the US Constitution. He adds that states are also free to withdraw those rights as long as they don’t fall below the constitutional minimum.

Others disagree. “The text of the Fourteenth Amendment’s guarantee of the ‘equal protection of the laws’ is sweeping and universal, securing equal rights under the law to all persons – and that includes gay men and lesbians,” Elizabeth Wydra, chief counsel of the Constitutional Accountability Center in Washington, said in a statement.

“I am confident that if the Supreme Court decides to accept this case, and the justices apply the Constitution’s text and history faithfully, the court will uphold the lower court’s decision,” she added.

Cooper’s brief also attacked the appeals court’s conclusion that there is no rational basis for a state to recognize opposite-sex marriages but not same-sex marriages. The appeals court said gay and lesbian couples are just as qualified as heterosexual parents to raise children.

The Prop. 8 legal brief said there is a critical difference between same-sex and opposite-sex couples. Opposite-sex couples are capable of producing children even in casual sexual encounters – and it is in society’s interest to encourage such couples to stay together, care for, and raise their children.

The traditional definition of marriage furthers society’s interest in responsible procreation and child-rearing, Cooper said. “It is plainly reasonable for the People of California, like virtually every society throughout human history, to maintain a unique institution to address the unique challenges posed by the unique procreative potential of sexual relationships between men and women,” he wrote.

Lawyers for ProtectMarriage.com, the Prop. 8 advocacy group, said they were looking forward presenting the case to the high court.

“Marriage is a universal good that has been honored by diverse cultures and faiths for the entire history of Western Civilization,” Austin Nimocks of the group Alliance Defending Freedom said in a statement.

“The democratic process and the most important human institution – marriage – shouldn’t be overthrown based on the demands of Hollywood activists,” he said.