The Washington Post-ABC News survey said 53 percent of the respondents agreed gay marriage should be legal compared to just 36 percent who agreed five years ago.
Proponents of gay marriage called the shift a reflection of the nation's increased understanding of the relatively benign nature of same-sex unions and reflective of similar recent polls.
But opponents of legal same-sex marriages said the poll question was slanted because it asked if such marriages should be legal or illegal, which could cause respondents to think homosexuals should be thrown in jail for getting hitched.
"It's absurd," said Brian Brown, president of the National Organization of Marriage, who noted that state ballot measures legalizing same-sex marriage fare poorly on the ballot. "The people of this country have not changed their opinion about marriage."
The telephone poll was conducted March 10-13 and surveyed 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.