The Gallup poll says 50 percent want to put gay couples on the same legal footing as straight couples, while 48 percent oppose legal same-sex marriage. Still, Gallup says this is a dip in support from last year's 53 percent.
The poll was released amid a flurry of debate about support for gay marriage in the White House and on the day that North Carolina, a critical election state, votes on a constitutional amendment banning same-sex unions.
Presidnet Barack Obama was forced to double down on his personal opposition to same-sex marriage after Vice President Joe Biden said Sunday he was comfortable with gay men and women marrying - a remark that falls a bit short of full-throated support.
Meanwhile, proponents of same-sex marriage appear to have failed to rally enough opposition to derail a North Carolina ballot measure that would change the constitution to prohibit any legal recognition of a union other than marriage between a man and a woman.
The poll, conducted from May 3-6, shows clear divisions among the parties and religious groups over same-sex marriage. Democrats and independents make up the bulk of same-sex marriage supporters and Republicans are mostly united in their opposition.
Fewer than a quarter of Republicans -- 22 percent -- say same-sex marriage should be legal. A majority of Democrats (65 percent) and political independents (57 percent) back legal recognition of gay couples.
Outside of political parties, there are also differences in opinion among religious Americans. Protestants oppose same-sex marriage 59-38 percent, but a majority of Catholics say gay couples should get full marriage rights, 51-47 percent.
Support was also dependent on how much Americans attend religious services. Regulars mostly oppose same-sex marriage, 67-31 percent; those who attend every now and then are more divided on the issue, with 53 percent against and 45 percent in favor. For those who rarely or never go to services, gay marriage drew 67 percent support.