American support for same-sex marriage is rising steadily - and conservative Protestants seem to be the last religious group holding out against it, a large new poll suggests.

Nearly six out of 10 white evangelicals were against gay marriage, and just over five out of 10 black Protestants opposed it, the Public Religion Research Institute announced Wednesday.

But among Catholics and members of so-called "mainline" Protestant churches, more people favored gay marriage than opposed it.

People not affiliated with any religion back gay marriage by a 4-to-1 ratio, the 2010 American Values Survey found.

Among all Americans, backing for allowing gay and lesbian marriages rose from 29 percent in 2008 to 37 percent today.

And a majority of young people back gay marriage, their survey found - 52 percent of 18-29 year-olds are in favor, while another 23 percent support civil unions.

The results mirror similar findings from another recent survey.

The Pew Forum on Religion & Public Life, another Washington think tank, found last week that 42 percent of Americans favored gay marriage, while 48 percent opposed it.

It was the first time in the 15-year history of the Pew Forum asking the question that opposition to gay marriage fell below 50 percent, the think tank said.

Pew asked if people favored gay marriage or opposed it. The Public Religion Research Institute asked a slightly different question: whether respondents favor gay marriage, civil unions, or opposed both.

The institute revealed its findings as it unveiled new information from its 2010 American Values Survey, a telephone poll of 3,013 American adults conducted from September 1-14.

Initial findings from the survey came out last week, and suggested that the Tea Party and the Christian conservative movement have more in common than conventional wisdom held.

Americans are aware that their views on gay marriage are shifting, they told the Public Research Religion Insititute.

About two out of 10 said they had become more supportive of gay and lesbian rights over time. About one in 20 said they had become less so, while 73 percent said their views had not changed.

Views on abortion, another hot-button "moral" issue, are more stable, the survey found.

Seven out of 100 people said they had become more supportive of abortion rights. Exactly the same number said they had become less so, while 85 percent said their views had not changed.

White evangelical Protestants, the group most opposed to gay marriage according to the American Values Survey, make up just over a quarter of the U.S. population, the Pew Forum's U.S Religious Landscape Survey says.

The American Values Survey confirmed that there's a sharp political divide between black and white conservative Protestants, despite their agreement on gay marriage.

Seven out of 10 white evangelicals are Republicans, while nine out of 10 black Protestants are Democrats.

More than six out of 10 white evangelicals say they would be less likely to vote for a candidate who supports abortion rights, while about three out of 10 black Protestants say that.