On an historic day for gay rights in America, the U.S. Supreme Court Wednesday snatched away from the federal government the power to ban same-sex marriages and gave advocates of marriage equality a powerful tool to overturn laws that define marriage as only between a man and a woman.

In a pair of 5-4 rulings, the court shifted the contentious struggle to define marriage to the states, where 31 of them — including Ohio — have passed constitutional amendments banning the legal recognition of same-sex unions.

In Ohio, the decision is likely to energize opponents of the 2004 same-sex marriage ban who are trying to gather signatures to place a repeal on next year’s ballot. It also injects some doubt into whether the current Supreme Court would approve the Ohio ban.

With a huge boisterous crowd waiting outside the court, the justices struck down a 1996 federal law known as the Defense of Marriage Act that defined marriage as between a man and a woman. Because of that definition, legally married gay couples were denied a host of benefits, including Social Security Death benefits when a spouse died.

In a companion ruling, the justices dismissed a challenge brought by supporters of a California referendum in 2008 that banned same-sex marriage. With their ruling, the justices let stand a 2010 decision by a federal judge in California that the referendum violated the U.S. Constitution, thus allowing same-sex marriage to resume in that state.

The rulings are a major victory for supporters of same-sex marriage. Even though the court’s decision allows the states to determine their marriage laws, it is clear that five justices — Anthony Kennedy, Ruth Bader Ginsburg, Stephen H. Breyer, Elena Kagan and Sonia Sotomayor — have deep reservations about state or federal laws that ban same-sex marriage.

Marc Spindelman, a law professor at the Moritz College of Law at Ohio State University, said while the ruling appears to order the federal government to simply defer to states that allow same-sex unions, the ruling’s broad language gives advocates a “powerful new tool” that may help them make the case against state bans of gay marriage.

The ruling “provides a new legal basis for saying state bans on same-sex marriage are unconstitutional,” he said.

In striking down the 1996 federal law, Kennedy concluded the government had created a “second-tier marriage’’ for gay couples married in 12 states and District of Columbia which recognize same-sex marriage.

“The differentiation demeans the couple whose moral and sexual choices the Constitution protects and whose relationship the state has sought to dignify,’’ Kennedy wrote. “And it humiliates tens of thousands of children now being raised by same-sex couples.’’

In his dissent, Justice Antonin Scalia warned that “by formally declaring anyone opposed to same-sex marriage an enemy of human decency, the majority arms well every challenger to a state law restricting marriage to its traditional definition.’’

Chief Justice John Roberts and Justices Samuel Alito and Clarence Thomas also dissented.

The Defense of Marriage Act, passed during the Clinton administration, was the reason the Internal Revenue Service informed Edith Windsor, 84, of New York City that she owed $363,063 in federal estate taxes after the 2009 death of her female partner, Thea Spyer.

Windsor and Spyer had been legally married in Canada in 2007, but their marriage was not recognized by the U.S. government, and therefore she could be denied Social Security death benefits and forced to pay estate taxes. Had Windsor been a man, she would have received Spyer’s estate tax-free.

In the second case, a completely different judicial lineup concluded that supporters of the California ban on same-sex marriage had not shown that they had been harmed by lower court’s decision in 2010 to invalidate the referendum.

Roberts, who wrote the opinion, was joined by Kagan, Breyer, Ginsburg and Scalia, while Kennedy, Thomas, Sotomayor and Alito dissented.

Sen. Sherrod Brown, D-Ohio, who voted against the Defense of Marriage Act when he was a House member, called the rulings “great news for Ohio.”

Sen. Rob Portman, R-Ohio, who voted for the act but who endorsed gay marriage earlier this year after his son Will came out as a gay, said he had hoped the federal law would be repealed by Congress rather than the high court.

“I believe that gay couples deserve the opportunity to marry, but I believe change should come about through the democratic process in the states, through the process of citizen persuading fellow citizen that civil marriage should be allowed,” he said.

Defense Secretary Chuck Hagel hailed the decision, saying the Pentagon “intends to make the same benefits available to all military spouses — regardless of sexual orientation — as soon as possible.’’

Hagel added that the “ruling helps ensure that all men and women who serve this country can be treated fairly and equally, with the full dignity and respect they so richly deserve.’’

In Ohio, the reaction was more mixed.

“I’m very disappointed in the Supreme Court ruling,” said Alice Martin of West Milton, 74. “I am hoping they would stand by my scripture and stand by the Bible. The Bible may have been written 2,000 years ago, but marriage is still between a man and a woman, and homosexuality is a sin. We were a Christian nation at one time, and the sad part is our nation has gotten away from our Lord and savior.”

Steve Stiglich, 60 of Fairborn, also objected on religious grounds. “It’s another step in the wrong direction away from God’s word,” said Stiglich, the associational missionary of the Greater Dayton Association of Baptists. “This is unfortunately the way the world is moving.”

Crowds began lining up in front of the Supreme Court Tuesday night, with many camping out in hopes of witnessing history.

“It was insane,” Grace Reinke, a student at Louisiana State University, said about the energy outside the building. “I’ve probably never seen anything like it in my life.”

Not everyone in the crowd was thrilled with the rulings.

The Rev. Rob Schenck, chairman of the Evangelical Church Alliance, one of the oldest associations of independent clergy in the nation, said his organization opposes “the social redefinition of marriage.”

He said “biblical truth does not change by the vote of any majority, including a majority of this Court.”