By Steve Gorman

LOS ANGELES (Reuters) - Gay marriage opponents asked the Supreme Court on Tuesday to uphold a California ban on same-sex matrimony that was struck down by two lower courts as a violation of the Constitution.

The request from backers of Proposition 8, the voter-approved state constitutional amendment defining marriage as exclusively between a man and a woman, comes a week after the high court was asked to review a Massachusetts case challenging the federal Defense of Marriage Act, which defines marriage the same way.

The two petitions move the politically charged issue of marriage rights for gay men and lesbians one step closer to a potential first-time review by the Supreme Court in the weeks before November's U.S. presidential election.

President Barack Obama turned gay marriage into a 2012 campaign issue in May when he came out in support of the right of same-sex couples to wed. His Republican opponent, former Massachusetts Governor Mitt Romney, disagrees.

The Supreme Court could agree to hear the California and Massachusetts cases in its next session, which begins in October, putting the court on track to issue a ruling within a year.

Alternatively, the high court could decline to hear either or both cases.

Refusing to weigh in on the fight over California's Prop 8 would keep intact lower-court rulings nullifying it but leave unresolved the broader question of whether similar gay marriage bans in other states would survive a constitutional challenge.

California, the most populous state, joined the vast majority of U.S. states in outlawing same-sex marriage in 2008 when voters passed Prop 8, overriding a state Supreme Court decision six months earlier that briefly legalized gay marriage.

The state high court, however, later ruled that 18,000 same-sex weddings officiated between May and November of 2008 would remain legal.

Gay rights advocates subsequently brought suit against Prop 8, and a San Francisco-based federal judge struck down the measure in a landmark 2010 decision that was upheld in February by the 9th U.S. Circuit Court of Appeals. The appeals court declined to reconsider the matter in June.

However, the California measure restricting marriage to heterosexual couples remains in effect until the legal challenge to Prop 8 runs its course, barring further weddings between gay men and lesbians in the state in the meantime.

DEFINITION OF MARRIAGE

The majority opinion in February's 2-1 decision by the 9th Circuit held that California's Prop 8 ban did not further the goal of "responsible procreation," which was at the heart of the argument made by supporters of the measure.

"Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California, and to officially reclassify their relationships as inferior to those of opposite-sex couples," the court ruled.

In focusing specifically on Prop 8, the appeals court left unaddressed whether marriage is a fundamental right available to homosexual and heterosexual couples alike.

Some legal experts have predicted that the narrow ruling would lead the U.S. Supreme Court to either limit its own review to the California law or refuse to consider the case altogether.

In its petition for high-court review, Prop 8 supporters argued that the U.S. Constitution leaves the definition of marriage up to individual states to determine.

"Whether the Constitution requires California to eliminate the most longstanding, universal and fundamental institution -- marriage consisting of one man and one woman -- is a question that should be settled by the Supreme Court," said Andy Pugno, general counsel for Prop 8's defenders.

David Boies, co-counsel for Prop 8 foes, said his side opposed the petition because "gay and lesbian Californians should not have to wait any longer to marry the person they love."

But he said gay rights advocates welcome Supreme Court review of the case, adding that the petition "presents the justices with the chance to affirm our Constitution's central promises of liberty, equality and human dignity."

If the Supreme Court does take the case, the outcome could well hinge on Justice Anthony Kennedy, a Republican-appointed conservative seen as a key swing vote. He has written two important decisions that came down on the side of same-sex couples, though he has not explicitly endorsed gay marriage.

Six U.S. states and the District of Columbia have legalized gay marriage but 30 have banned it.

Last week, Massachusetts' attorney general, Martha Coakley, petitioned the Supreme Court to uphold a federal appeals court decision striking down parts of the federal Defense of Marriage Act.

The 1st U.S. Circuit Court of Appeals in May affirmed a lower-court ruling that gay and lesbian couples who were legally permitted to wed in Massachusetts had been unconstitutionally denied federal benefits because of the Defense of Marriage Act, including the right to file joint income tax returns or collect Social Security retirement benefits.

(Editing by Cynthia Johnston, Cynthia Osterman and Lisa Shumaker)