Maryland is set to become the first state to back same-sex marriage at the ballot box, with a poll released Thursday showing voters overwhelmingly in support of a new marriage law, 57 to 37 percent.

The survey from Public Policy Polling, a Democratic-aligned firm, showed a surge of support for keeping same-sex marriage legal in the state since Gov. Martin O'Malley signed the legislation in March. PPP credits the 20-point spread in support of same-sex marriage to a shift among black voters, most of whom now plan to back the law this November.

More than half of black voters polled in March planned to vote down same-sex marriage; a poll at the time showed the law being nonetheless upheld, 52 to 44 percent. In the two months since the law passed, black support has flipped, with 55 percent planning to vote for the law, with 36 percent opposed. Maryland is nearly 30 percent black, according to the 2010 Census, one of the highest proportions in the country.

The poll surveyed 852 likely voters between May 14 and 21 -- after President Barack Obama voiced his personal support for allowing gay men and women get married, with each state making its own decision on the matter.

Maryland voters were already prepared to support marriage equality at the polls this fall even before President Obama's announcement, PPP's polling memo said. But now it appears that passage will come by a much stronger margin.

Same-sex marriage has yet to prevail when placed on the ballot; North Carolina this month passed a constitutional amendment limiting legal unions to only marriage between a man and a woman. Over the last decade, gay marriage was voted down in the Democratic, liberal-leaning states of California, Maine and Oregon.