Political observers are not expecting such suspense in the Senate. A similar bill passed last year, 25- 21, with one member not voting.
Evans said her group and others are still working to be sure minds have not changed since last February, when the chamber voted.
"It'll be a very close vote," predicted Sen. President Thomas V. Mike Miller. He said he wants to wrap up the bill before the body turns its attention to the budget, which he said will also be controversial.
"This is a very emotional issue," Miller said.
Senate Judicial Proceedings Committee Chairman Brian Frosh has set a committee vote for noon, and said he plans to take up the House version of the bill. Should supporters resist amendments in committee and on the floor, passage in the Senate would be the last step before O'Malley signs it into law.
If opponents attempt a filbuster, Senate rules require 29 votes to end it. Last year, Miller worked to ensure the measure came to a vote, voting to end debate even though he opposes same-sex marriage.
If the bill passes in both chambers, Maryland would be the eighth state to approve gay nuptials. Washington, D.C. also permits them.
Opponents say they'll keep fighting. The Maryland Marriage Alliance, a coalition of faith leaders opposing the measure, sent out an email Monday urging followers to contact their lawmakers. "We must look now to the Senate," wrote the Rev. Derek McCoy.
Mary Ellen Russell, the executive director of the Catholic Conference, pledged that her group will "leave no stone unturned in the Senate" in its effort to defeat the legislation.
She said that over the past year, senators have "heard from their constituents" on the issue. "The impact of that input could potentially cause members to reconsider their vote," Russell said.
The Catholic group held its lobby night Monday in Annapolis, where hundreds were to meet with their lawmakers.
Russell said they will also take time to thank members of the House of Delegates who "stood strong" and voted against the measure. "They deserve a lot of credit," she said.
The Civil Marriage Protection Act would remove a 39-year-old law that defines marriage as between a man and a woman. The law would go into effect January 2013, a concession made on the House floor by delegates who wanted to be sure that there would be ample time for an expected statewide referendum on the issue.
Talk of the signature-gathering process has begun. To trigger a referendum, opponents will have to gather about 55,000 signatures, an amount equal to 3 percent of the number of voters in the most recent gubernatorial election.