Support for same-sex marriage among Americans has jumped significantly in the past year to an all-time high of 58 percent, according to a new Washington Post-ABC News poll.

That number reflects a remarkable — and remarkably fast — turnabout in American public opinion on one of the most emotionally raw and politically divisive issues of the past decade. As recently as 2010, opponents of same-sex marriage outnumbered supporters. As recently as 2006, they outnumbered them by a double-digit margin, 58 percent to 36 percent.

Seven years later, that picture has turned upside down.

The change is apparent across the board, with Americans of all political stripes and age groups becoming increasingly supportive of gay marriage. Fully 81 percent of adults between the ages of 18 and 29 support such unions, and although support dips to 44 percent among those 65 and older, both of those figures are highs.

Most Republicans continue to oppose same-sex marriage, but among Americans younger than 50, a slim majority of Republicans and GOP-leaning independents now support it.

Public support for same-sex marriage hits a new high

On Monday, there were signs that both parties’ establishments were trying to catch up to this shift in beliefs. Republican leaders, performing an “autopsy” of last fall’s election defeats, said they fear that the GOP’s position on same-sex marriage is driving some young voters away.

And among Democrats, former secretary of state Hillary Rodham Clinton appeared in a Web video to endorse legalizing same-sex marriage.

“I support marriage for lesbian and gay couples,” Clinton says in the video, released by the Human Rights Campaign, an advocacy group. “I support it personally and as a matter of policy and law.”

During her presidential run in 2008, Clinton endorsed same-sex “civil unions” but not marriage. But her shift does not make her the first major potential contender for the 2016 Democratic nomination to signal support for gay marriage.

In fact, it makes her the last. She follows Vice President Biden, Maryland Gov. Martin O’Malley and New York Gov. Andrew M. Cuomo.

Peter Sprigg of the Family Research Council, one of the most prominent of the groups that oppose same-sex marriage, said: “I don’t believe we’re losing the battle. We’ve certainly lost some ground in public opinion.” His group believes that traditional marriage — between one man and one woman — is dictated not just by civil and religious laws but by nature as the institution best suited to raising children.

“Obviously, we’ve lost some ground in terms of the states that have” legalized same-sex marriage, Sprigg said. But he said he is optimistic: “I think we’ll continue to have a lively and vigorous debate, state by state.”

Next week, the U.S. Supreme Court will hear arguments in two closely watched cases that are challenging legal limits on same-sex marriage.

One case involves California’s Proposition 8, a voter-approved measure that bars such marriages in the state. The other is a challenge to the 1996 Defense of Marriage Act. That law withholds federal recognition of same-sex marriages performed in the nine states, and the District of Columbia, where they are legal.

Supporters of same-sex marriage said poll results like the ones released Monday show that, in time, their side will have the support to prevail.

“Young people have grown up in the middle of a conversation about who gay people are and why gay marriage matters to loving and committed gay couples,” said Evan Wolfson, president of the advocacy group Freedom to Marry. “They therefore are not impeded by stereotypes and discomfort, but rather got to think about real people and real values.”

The poll also showed a broader shift in American attitudes about homosexuality. Two decades ago, fewer than half of all Americans said being gay was an identity people are born with, not a choice; today, a sizable majority, 62 percent, says so.

Often, that belief engenders another. Nearly three-quarters of those who believe that homosexuality is something innate also support gay marriage.

“We all feel like you were born that way. Being gay is not a choice,” said Gino Tomasetti, 35, of Southborough, Mass. Tomasetti said he’d never been to a same-sex wedding, although they have been taking place in Massachusetts since 2004. He didn’t know anybody who had been to one. But he said he supports the idea strongly.

“As long as you know that it’s not a choice,” said Tomasetti, who works in network administration, “I don’t see why we should punish people for that.”

Among those who believe that homosexuality is a choice, about two-thirds think that same-sex marriage should be illegal.

“People ain’t born that way, like they say they are. People choose to be that way,” said Jason Smith, 37, a machinist in Jasper, Ga. He opposes same-sex marriage but worries that it is becoming more accepted in mainstream culture.

“You see a lot more of it that you used to,” Smith said. “It makes kids think it’s okay.”

The Republican Party’s 2012 platform called for a constitutional amendment to ban same-sex marriage. But Monday, in their report examining last fall’s election, party leaders described a “generational difference within the conservative movement about issues involving the treatment and the rights of gays.”

The document calls for Republicans to embrace different points of view, “instead of driving around in circles on an ideological cul-de-sac.”

One of the report’s authors is Glenn McCall, a national committeeman from South Carolina. He told reporters that gay rights was an issue that “turned off” younger voters.

“For many younger voters, this issue is a gateway into whether the party is a place they want to be,” McCall said. “If our party isn’t welcoming and inclusive, young people and increasingly others will continue to tune us out.”

Rachel Weiner and Robert Barnes contributed to this report. Cohen is director of polling for Capital Insight, the independent polling group of Washington Post Media.

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