Maryland Governor Martin O'Malley has signed the historic same-sex marriage bill into law, making Maryland the seventh state along with D.C. to allow same sex marriage and the first on the East Coast below the Mason-Dixon line.
Maryland's House passed the bill on Feb. 17 by a narrow margin of 71-67 in a move some delegates called the most important vote they ever cast. The Senate passed the bill on Feb. 23.
Opponents have vowed to bring the measure to referendum on the November ballot and began the petition process Friday, a day after the state Senate approved it in a 25 to 22 vote.
They will need to collect 55,736 valid signatures of Maryland voters to put it on the ballot.
Gay marriage advocates said they will fight to make sure the law is upheld and that they think it is inappropriate to leave the issue to the discretion of voters.
When a gay marriage bill fell short in the legislature last year, black pastors were given much of the credit for pressuring lawmakers to oppose it. Catholic leaders also expressed vocal opposition to that measure, which was pulled from the floor of the House as leaders realized if fell short of the needed votes.
Six other states and Washington, D.C., recognize gay marriages. Washington State's new gay marriage law is scheduled to take effect in June, but voters there are also 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.
Meanwhile, about 30 states have constitutional amendments that seek to prohibit gay marriage, most by defining marriage as between a man and a woman.
The Associated Press contributed to this story.