The declaration by Sen. James C. Rosapepe came shortly after a Senate committee approved an amended version of the Religious Freedom and Civil Marriage Protection Act, setting up a debate in the full Senate next week.
Senate President Thomas V. Mike Miller said the chamber could hold a final vote on the bill within the next 10 days.
If the narrow majority holds together through what is expected to be a contentious floor debate — and the possibility of amendments — the legislation would move to the House of Delegates, where supporters believe it would pass.
Gov. Martin O'Malley has said he would sign such a bill. Opponents could then petition for a referendum to overturn it, which would give voters the final say.
Passage would make Maryland the sixth state to recognize same-sex unions. Gay marriage is also legal in the District of Columbia.
Rosapepe, a Prince George's County Democrat, said in an e-mail to constituents that he intended to vote for the bill because it now contains greater protections for religious groups that don't want to participate in same-sex unions.
The contentious legislation gained momentum all week as undecided senators declared their support. Miller, who does not support gay marriage, predicted the Senate debate will be fierce, but he believes it will pass.
Miller said he wants "everyone to have their say" next week.
"When it appears people are repeating themselves, then we will take a cloture vote," he said. Senate rules require 29 votes to cut off debate and call an up-or-down vote. Advocates believe they have those votes.
Previous attempts to recognize same-sex marriage had never before made it out of the Judicial Proceedings Committee. But Democratic gains in November made the panel more amenable to the legislation.
"This is a historic piece of legislation," said committee Chairman Brian Frosh, a longtime supporter of gay marriage.
'I think it is a good thing if we encourage people to take care of each other."
The bill would repeal the definition of marriage as the union of a man and a woman. It would not force churches, synagogues or mosques to perform or celebrate gay weddings.
At a long and contentious hearing last week, opponents said the bill did not include adequate protections for groups affiliated with religious institutions that did not wish to support same-sex marriage. For example, they said, it would not shield the Knights of Columbus, a Catholic fraternal organization, from discrimination claims if it did not wish to rent out a hall for a gay wedding.
Rosapepe spent about an hour behind closed doors with Frosh and Sen. Jamie Raskin, both of whom sponsored the bill, before the committee meeting Thursday.
Raskin, a Montgomery County Democrat, had argued that further protections were unnecessary. But at the committee meeting, he proposed the amendment to more explicitly shield religious organizations, societies and nonprofits.
He said jokingly that the change could be called the "Raskin-Rosapepe amendment."
The influential Maryland Catholic Conference, the lobbying arm of the state's Catholic bishops in Annapolis, noted the change but restated its opposition.
"Our opposition to this bill does not rest on a simple concern for the interests of religious institutions only," the conference said in a statement. "The bill continues to provide no protections for an individual's religious freedoms, such as those of a clerk forced to issue a marriage license to a same-sex couple.
"More importantly, our fundamental concern about redefining marriage is for the sake of our whole society, and particularly for children and their elemental desire to know, and ideally to be raised and loved by, their biological mother and father."
But the change was enough to sway Rosapepe, who is Catholic. He had not previously discussed his position publicly.
"I intend to vote for the bill as it was reported out of committee with a strengthened conscience clause to respect the views of religious denominations which do not recognize same-sex marriage," he wrote in his e-mail to constituents.
The bill might still be amended during the floor debate, which could threaten the support of the narrow majority. Rosapepe acknowledged the possibility in his e-mail: "I don't know what other amendments may be proposed on the Senate floor but will keep your concerns in mind as we consider them."
Senate Majority Leader Rob Garagiola, the bill's lead sponsor, attended the committee meeting.
Sen. James Brochin offered an amendment to transform the same-sex marriage bill into a civil unions bill. The committee rejected the idea by a vote of 10-1.
The bulk of the debate centered around Raskin's amendment, which says groups affiliated with religious institutions would not have to perform same-sex weddings, and they also would not have to "celebrate" them.
In what will undoubtedly be a preview of the Senate debate, some argued that the change would allow widespread discrimination against gay couples.
"It is incredibly broad," said Sen. Robert Zirkin. "You are inviting all sorts of lawsuits with this language."
The Baltimore County Democrat argued that the provision could allow a group to exclude same-sex couples who, for example, wanted to simply hold an anniversary party.
Raskin agreed with the example. The point of the provision, he said, was to extend the same freedoms of expression to religious groups that he is supporting for gay couples.
Committee passage of the bill had been expected: All seven of the supporters had said they would vote for the bill.
After the vote, even opponents seemed resigned to fighting an uphill battle.
"The public opinion has changed on this issue," said Senate Minority Leader Nancy Jacobs, a Republican representing Harford and Cecil counties. She said she is going to vote against the bill.