WASHINGTON — The US Supreme Court could decide later this year whether to take up the issue of gay marriage, after a group of lawmakers asked it be deemed unconstitutional, a court source said Saturday.

The subject is a hot-button issue that soared to the forefront of the political debate in May when Barack Obama became the first US president to say publicly that he was in favor of same-sex marriage.

On Friday, the Republican-led Bipartisan Legal Advisory Group filed a petition with the US high court, asking it to say that a law defining marriage as a union between a man and a woman is consistent with the fifth amendment of the US Constitution, which calls for equal protection under the law.

The lawmakers also asked the nine Supreme Court justices to review an appellate court ruling which said that the 1996 Defense of Marriage Act law is unconstitutional, according to the petition, a copy of which was seen by AFP.

The Bipartisan Legal Advisory Group (BLAG) consists of three leading Republican lawmakers and two key Democrats, but one of those Democrats, House Minority Leader Nancy Pelosi, said she did not support the move.

In a statement, Pelosi said House Speaker John Boehner and House Republicans "decided to waste more taxpayer funds to advance a position rejected by four different courts and to defend discrimination and inequality before the highest court in the land."

The nine justices of the Supreme Court, who are now on vacation after the conclusion of their annual session, will reconvene in late September. At any point thereafter, they could decide on whether to take up the case.

The petition is the first on gay marriage to come before the high court.

"This case calls out for the Court's review," reads the BLAG petition, calling on the Supreme Court to reject the May 31 ruling of a federal appeals court in Boston that DOMA was unconstitutional and discriminatory.

The decision by the Boston appeals court affirmed a lower court's ruling that the law is unconstitutional, saying that DOMA denies homosexual couples the same rights afforded to heterosexual couples.

However, the BLAG petition said the ruling "invalidates an Act of Congress, conflicts with the decisions of this (Supreme) Court and numerous other courts of appeals, and embraces an entirely novel approach to constitutional equal protection analysis."

"Only this Court can settle this matter definitively. Unless and until this Court decides the question, the executive branch will continue to attack DOMA in the courts," the BLAG petition read.

On June 5, a US appeals court in California upheld its decision to strike down a law banning gay marriage in the state. That case is also expected to be taken before the Supreme Court.

So far, six states plus the national capital Washington DC have legalized gay marriage. Two others, Washington state and Maryland, have voted in favor, but their laws have yet to come into effect pending referendums.

In May, Obama said the decision on whether to legalize gay marriage should be left to individual states.