Five states and the District of Columbia allow gay marriage, six states perform civil unions bestowing legal rights equivalent to marriage, and several states offer same-sex couples limited legal protections short of a formal union.
Thirty states have constitutional amendments banning gay marriage.
"This is the next frontier," Human Rights Campaign senior strategist Brian Ellner said of the same-sex marriage fight. "Marriage is the way society has traditionally recognized relationships."
Ellner, who's leading the push for marriage equality in New York, said that successful repeal of the military's "don't ask, don't tell" policy has given momentum to the effort and provided a blueprint for lobbying on the state level.
"It was important to have a bipartisan coalition to repeal "don't ask, don't tell," and I'm hopeful that that kind of coalition can come together again on marriage," he said. " Equality should not be a partisan issue, and more and more prominent Republicans like Laura Bush, Dick Cheney and Ted Olson are supporters of marriage equality."
President Obama, who has opposed extending marriage rights to gays and lesbians, also recently signaled that his views also might be changing.
"My feelings are constantly evolving," Obama told reporters in December about his position on gay marriage after Congress approved a repeal of "don't ask, don't tell."
"I have friends, I have people who work for me, who are in powerful long-lasting gay or lesbians unions.
"My baseline is a strong civil union that affords them legal protections," he said. "I recognize from their perspective, it's not enough."