A slim majority of Americans now support gay marriage, according to a new Washington Post-ABC News poll.

The results underscore the nation’s increasingly tolerant views about homosexuals, and parallel a string of recent legal and legislative victories for gay rights advocates.

Five years ago, at 36 percent, support for gay marriage barely topped a third of all Americans. Now, 53 percent say gay marriage should be legal, marking the first time in Post-ABC polling that a majority has said so.

“This is very consistent with a lot of other polling data we’ve seen and the general momentum we’ve seen over the past year and a half,” said Evan Wolfson, president of Freedom to Marry, a leading pro-gay-marriage group. “As people have come to understand this is about loving, committed families dealing, like everyone, with tough times, they understand how unfair it is to treat them differently.”

Opponents of same-sex marriage took issue with the poll, which asks respondents: “Do you think it should be legal or illegal for gay and lesbian couples to get married?” Brian Brown, president of the National Organization for Marriage, argued that the term “illegal” could be inferred to mean that violators could be imprisoned, which most Americans would consider harsh.

Brown, whose group is a prominent anti-gay-marriage group, noted that all 31 states that have put same-sex marriage on the ballot voted to ban it.

“The only poll that counts is a free and fair vote on the part of the people,” he said. “We’ve seen these biased polls time and time again — right before votes in which same-sex marriage is rejected. It’s absurd. The people of this country have not changed their opinion about marriage.”

Post-ABC News polls have used the same “legal or illegal” wording in every poll about same-sex marriage since 2003. Other surveys by the Pew Research Center, the Associated Press and CNN show similar trends.

In the new Post-ABC poll, the shift has been driven by several political and demographic groups whose support for such unions jumped sharply. Men, who previously were less supportive of same-sex marriage than women, now back it at the same rate. Support among college-educated whites, political independents and people who do not consider themselves religious also rose substantially.

Republicans, conservatives and white evangelical Christians remain the groups most opposed to legalizing gay marriage.

The survey also shows a shift in how intensely people feel on this issue. In the past, the number of Americans who felt strongly that gay marriage should be banned far outnumbered those who were passionate in their belief that it should be legal. That has balanced out, with 35 percent strongly against legal gay marriage and 36 percent strongly in favor.

Passionate opposition to gay marriage last year in part led voters in Iowa to oust three state Supreme Court justices who had joined in the unanimous decision to legalize same-sex unions in the state. Minnesota, Indiana, Pennsylvania and North Carolina are considering steps that would ban same-sex marriage in their states.

But those victories for opponents of same-sex marriage have lately been overshadowed by several defeats.

Last March, the District joined five states in allowing same-sex couples to marry. Later in California, a federal judge struck down that state’s voter-approved ban on same-sex unions. The issue is now before an appeals court and is likely to end up at the Supreme Court.

In the summer, a federal judge in Massachusetts struck down the federal government’s ban on recognition of same-sex marriages under the 1996 Defense of Marriage Act. This issue, too, will probably end up before the nation’s high court. In a major victory for supporters of gay marriage, the Obama administration announced earlier this month that it would no longer defend the statute in court.

Congressional Republicans have vowed to defend the law themselves and criticized Obama for elevating such a divisive social issue at a time the focus ought to be on the economy and jobs. This week, Democrats introduced bills to repeal the Defense of Marriage Act and noted how the tables have turned.

“What do I say to the idea that this is a wedge issue? I say ‘Hallelujah,’ ” Rep. Barney Frank (D-Mass.), who is gay, told reporters, according to the liberal Web site Talking Points Memo. “The fact that we’ve now evolved to the point where the Republicans are complaining about the fact that we introduced this bill because it causes them political problems is a great sign of progress. It used to be the other way around.”

The telephone poll was conducted March 10 to 13, among a random national sample of 1,005 adults. Results from the full poll have a margin of sampling error of plus or minus 3.5 percentage points.