The law's opponents submitted 122,481 signatures in favor of a referendum; 55,736 are required. If enough are verified as legitimate, as expected, Maryland will be in the center of a national debate on same-sex marriage, with groups on both sides preparing to spend millions.
The Rev. Derek McCoy, who heads the leading group opposed to the law, called the effort "absolutely exhilarating."
"Marylanders have a right to vote on this issue," said McCoy, executive director of the Maryland Marriage Alliance, after the group dropped off boxes of petitions outside the Maryland secretary of state's office.
The state board of elections has 20 days to review the signatures.
The marriage law would be the second on this fall's ballot. Opponents of a law to allow some illegal immigrants to pay in-state tuition at Maryland universities have won a referendum on that measure. A campaign is under way against a third law, the state's new congressional districting map.
Maryland's Democratic-led General Assembly passed a law in March legalizing same-sex marriage, but it does not go into effect until January 2013. The date was picked to give opponents time for a referendum.
Opponents collected petition signatures in every county in Maryland, according to a report they filed with the state. They found the most in Baltimore County, with 20,363 opposing the law. In Montgomery County, a Democratic stronghold where all but one lawmaker voted for same-sex marriage, 16,611 people signed, the second-highest number.
Three other counties accounted for more than 10,000 signatures each: Prince George's, Anne Arundel and Harford.
The effort far outpaced last year's successful initiative to trigger the referendum on the in-state tuition law. The coalition opposing that law submitted 47,288 signatures at this point in the process.
McCoy said recent endorsements for same-sex marriage by President Barack Obama and the NAACP had the unintended effect of energizing the opposition. "Every day we have had more and more momentum," McCoy said.
But gay rights advocates argue that public opinion is moving in their direction, citing the results of recent polling they commissioned.
"We expected them to make their numbers," said the Rev. MacArthur Flournoy, faith leader for Marylanders for Marriage Equality, a group that supports the law. "Once stories are told, people see that hearts and minds are changed."
McCoy dismissed the polling as "propaganda."
Once the state Board of Elections certifies signatures, both sides can form formal ballot committees. Those committees can accept unlimited donations for their campaigns, with their first financial disclosure reports due four weeks prior to the November referendum.
In a petition-campaign report filed Tuesday, the Maryland Marriage Alliance reported spending roughly $70,000 collecting signatures — and said it has about $10,000 left over.
Funds raised included a $25,000 check from the National Organization for Marriage, a national group that has poured millions into other states to defeat same-sex marriage laws. It provided an additional $48,000 worth of services.
The financial report showed that donations of $50 or less came from donors across the country — including Arkansas, California, Alabama, Ohio, Texas, Massachusetts, Rhode Island and Connecticut.
Later Tuesday afternoon, Del. Neil Parrott, a freshman Republican from Frederick County who led the in-state tuition repeal campaign, pulled up in his hatchback and dropped off an additional 12 boxes of petitions against same-sex marriage. His organization, MDPetitions.com, was paid $18,000 for help with the effort.
Parrott said last year's in-state tuition campaign felt "more spontaneous."
"This was more organized," he said. "It speaks well for the organization and for the energy opposing the law." He estimated that 20 percent of the signatures came from an online tool he developed last year for the tuition law drive.
Parrott is also heading up an effort to repeal the congressional map that the General Assembly passed during an October special session. He has until Thursday to submit an initial 18,579 signatures. He declined Tuesday to say how many names he has collected.