The United States is in the midst of a broad and rapid change in attitudes about gay marriage, with 55 percent now favoring full recognition of same-sex marriage and 40 percent opposed, according to a national survey released on Thursday.

The poll, conducted for a gay rights organization, found that support for same-sex marriage is highest among young adults, with 75 percent approval among those ages 18 to 29. Among that group, 58 percent said they are strongly in favor, compared with 13 percent of 18- to 29-year-olds identified as strongly opposed.

The only age group with a majority opposed to such recognition is those 65 years and older. Among that group, 55 percent said they disagree with gay marriage.

Pollsters said they are impressed by the speed with which public opinion in America is changing on the highly contentious issue.

“This is moving faster than any issue we’ve ever tracked,” said Alex Lundry of the market research firm TargetPoint. “This is the future talking to us.”

Mr. Lundry served as director of data science for the Mitt Romney presidential campaign. He teamed up with Dave Walker of the polling firm Greenberg Quinlan Rosner Research, which conducted the survey. It is based on the responses of 1,000 likely voters in the 2016 elections. They were questioned March 9 to 16.

“It is not just marriage that has changed,” Mr. Walker said at a news briefing. The findings are a result of changing attitudes about the gay, lesbian, bisexual, and transgender community as a whole, he said. It is a level of acceptance that he said was unimaginable a few years ago.

Other polls have documented the same shift. Research conducted by Gallup shows that opposition to gay marriage has evolved from 55 percent opposed in 2004 to 54 percent in favor now, according to its most recent data. The Gallup data show 43 percent opposed to gay marriage.

“What is really driving this is knowing gay people,” Walker said. He said personal contact with gay family members, gay friends, and gay neighbors is winning what activists call the battle for “hearts and minds.”

The survey asked prospective voters if they felt that over the past five to 10 years they have become more accepting or less accepting of gays and lesbians. Most respondents said they had not changed their views, but 29 percent said they have become more accepting. In contrast, 7 percent said they were less accepting of gays and lesbians.

In further documenting the generational aspect of shifting attitudes, the pollsters found that 42 percent of Republicans ages 18 to 29 favor same-sex marriage, compared with 25 percent of Republicans ages 50 and older.

In addition, the survey found that 40 percent of “conservatives” ages 18 to 29 support gay marriage, while 21 percent of conservatives 50 and older do.

Forty-five percent of young evangelicals support same-sex marriage, while 19 percent of those 50 and older do.

The survey was conducted on behalf of a new coalition of gay rights groups, Americans for Marriage Equality. The poll was commissioned in part to support an aggressive and growing litigation strategy to challenge and strike down barriers to same-sex marriage across the United States. The plan is to eventually get the issue before the US Supreme Court.

Currently, same-sex marriages are legal in 17 states and the District of Columbia. In contrast, 33 states restrict marriage to one man and one woman through bans enacted by statute or constitutional amendment.

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Lawyers have filed 57 lawsuits in state and federal courts in 27 states and Puerto Rico challenging the bans. They are representing 550 gay and lesbian plaintiffs seeking the right to marry.

Nine cases are now on appeal in six federal circuit courts of appeal.