BELTSVILLE, Md. (AP) — Maryland’s governor signed into law Thursday a bill to legalize gay marriage, although opponents vowed to rally voters to reverse the change this fall in a referendum that’s even anticipated by advocates of the new law.

“Religious freedom was the very reason for our state’s founding and at the heart of religious freedom is the freedom of individual conscience,” Gov. Martin O’Malley said before signing the legislation that made Maryland the eighth state in the U.S. to legalize gay marriage. The law takes effect in 2013.

Six states and the District of Columbia currently recognize gay marriages. The state of Washington also has legalized gay marriage — its law takes effect in June. Voters there also are expected to petition the measure to referendum this fall.

Maine legalized the unions for same-sex couples in 2009, but later that year became the only state overturn a such a law passed by a legislature. About 30 states have constitutional amendments that seek to prohibit gay marriage, most by defining marriage as between a man and a woman.

In order to put the measure on the November ballot, opponents of the new Maryland law will need to collect nearly 56,000 valid voter signatures, equivalent to 3 percent of the people who cast ballots in the 2010 gubernatorial election. Last week, opponents submitted draft language for a ballot referendum to overturn the gay marriage measure after it passed in the state legislature.

Gay marriage advocates are hoping that young voters — whom they expect to support their cause — will turn out for President Barack Obama in November’s elections, just as they did in 2008.

“I think Obama’s election turns out a number of different people,” said Sultan Shakir, campaign manager for Marylanders for Marriage Equality, a coalition of gay rights groups that worked to get the bill passed. ”(There is) a lot of attention around people who attend church, but there are plenty of other demographics who are going to be turned out.”

Some observers have noted the push to overturn the new law will heavily rely on members of black churches. Many African-American church leaders oppose gay marriage in the liberal-leaning state that’s nearly one-third black, and Obama’s re-election campaign is expected to drive many of their congregants to the polls. The Catholic Church, which has 1.2 million parishioners in Maryland, also has openly opposed the bill.

Black pastors were given much of the credit for pressuring lawmakers to oppose a gay marriage bill that fell short in the legislature last year. The measure was pulled from the floor of the House as leaders realized if fell short of the necessary votes. Opposition from black pastors in Maryland belies an overall political stance that routinely includes their endorsement of Democratic candidates and support of their agendas.

Over the weekend, some pastors at predominantly black churches were already using their sermons to shop the referendum effort to their congregations, asking members to sign up for email alerts, put their name on petitions and overturn the law come November.

A Sunday service at the Hope Christian Church in Beltsville was filled with murmurs of agreement as a spokeswoman for the Maryland Marriage Alliance rallied the mostly black congregation against the law.

“We will have the last say on how marriage will be defined in Maryland,” spokeswoman Dee Powell shouted repeatedly to the audience of several hundred.

Some churchgoers said they are bound by their faith to vote against gay marriage.

“It’s a personal value and opinion. It has nothing to do with President Barack Obama,” said 54-year-old DeBorah Martinez, who has attended Hope Christian for three years.

Proponents of gay marriage also are counting on religious leaders who support of the bill to influence their congregations and for labor unions to urge their members to vote to keep gay marriage legal. Some black pastors who supported the measure as a matter of civil rights appeared publicly with O’Malley, a Democrat, during the legislative debate.

The advocates said it is inappropriate to leave what they consider a civil rights issue to the discretion of voters.

“It’s sad to me that anyone would think that it’s OK to put up the rights of a minority to a popular vote,” said Lisa Polyak, chairwoman of the board of directors for the gay rights organization Equality Maryland. “We have children, we have lives, we have jobs and we just want to go about them with integrity.”

Donald Norris, chairman of the department of public policy at the University of Maryland, Baltimore County, said black churches could heavily influence the referendum, but liberal voters who come out to support Obama could offset the votes against same-sex marriage.

A number of factors could tip the vote on a referendum, Norris said. For example, a weak Republican presidential candidate could mean conservative voters stay home and don’t cast ballots against the law.

“It’s going to really depend upon a variety of things that are going to happen between now and November,” Norris said.

Babatunde Adedayo, a 29-year-old from Upper Marlboro, said the president and his stance on gay marriage will likely influence his peers in November. Obama supports civil unions, but has not endorsed marriage for same-sex couples

“I think this affects every facet of our culture,” Adedayo said after the service at Hope Christian. “As a black African-American in America, it is something the black church takes seriously and depending on Barack Obama’s stance on this, it will affect a lot of people.”

Copyright 2012 The Associated Press.