The Pew Research Center said support for civil unions has risen to 57 percent, up from 54 percent a year ago and 45 percent when the question was first asked by Pew in 2003.
Views on legalizing same-sex marriage remained almost unchanged from last year, with 53 percent opposed and 39 percent in favor, the center said.
Supporters of same-sex marriage were divided over the best way to pursue legalization, according to the survey. Forty-five percent of them favored pushing hard to legalize it as soon as possible, while 42 percent said pushing too hard for swift legalization might trigger a backlash against gays and lesbians.
There was more support for same-sex marriage among women, adults under 30 and college-educated people than there was among men, older adults and those who did not attend college.
Asked about homosexual behavior, 49 percent said it is morally wrong, 9 percent said it is morally acceptable and 35 percent said it is not a moral issue.
The telephone survey of 4,013 adults was conducted in August. The margin of error for the full sample was 2 percentage points.
Same-sex marriage is legal in Massachusetts, Connecticut, Iowa and Vermont, and will start in New Hampshire in January. A referendum in Maine on Nov. 3 will determine the fate of a same-sex marriage bill passed by the Legislature in May, and the city council in Washington, D.C., is expected to approve same-sex marriage soon.
Several other states offer civil unions, domestic partnerships or other arrangements that provide varying degrees of marriage-like rights to same-sex couples, including California, Hawaii, Washington, Oregon, Nevada, Colorado and Wisconsin.
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