A majority of Maryland voters say they support gay marriage.

Maryland Governor Martin O'Malley in March signed a gay marriage bill approved by lawmakers into law, making Maryland the eighth state to legalize such unions.

Opponent have vowed to put the law up for a vote in November. Nearly 56,000 valid signatures, one-third of which would need to be submitted by May 31, with the remainder by June 30, are needed to put the measure on the ballot. The campaign is being organized by Maryland Marriage Alliance, which is supported by the National Organization for Marriage (NOM).

According to a Public Polling Policy (PPP) survey of 852 likely voters conducted between May 14 and 21 and commissioned by gay marriage advocate Marylanders for Marriage Equality, 57 percent of respondents said they would vote to keep the law, a 5 point increase from March. Thirty-seven percent said they would vote to repeal it, a 7 point decrease.

The increase in support is coming almost entirely from a major shift in opinion among black voters.

In March, only 39 percent of African American voters said they supported gay marriage. The new poll found 55 percent of black voters in favor.

The shift comes two weeks after President Barack Obama announced his support for equal marriage rights for gay couples. Over the weekend, the NAACP followed Obama in announcing its support.

“Things are moving in Maryland,” Josh Levin, campaign manager for Marylanders for Marriage Equality, said in a statement. “We're approaching a supermajority who want to uphold the state's new marriage law. The message of stronger families and greater fairness is resonating, and we're confident Maryland will be the first state to win a ballot measure on marriage equality and religious freedom.”

“The President's backing of marriage equality has added to our momentum – and his being on the November ballot also helps us,” Levin added. “Younger voters, who are overwhelmingly supportive, are much more likely to turn out in a presidential year.”

“While the media has been focused on what impact President Obama's announcement will have on his own reelection prospects, the more important fallout may be the impact his position is having on public opinion about same-sex marriage itself,” PPP said in releasing the poll's findings.

(Related: Support for gay marriage increases among black Pennsylvanians.)