BOSTON — In a suspense-filled final day of the legislative session, Massachusetts lawmakers kept alive a proposed constitutional amendment yesterday that would put a stop to gay marriage in the only state that allows same-sex couples to wed.

The vote came after weeks of mounting legal and political pressure on legislators from both sides in the debate.

With a combination of parliamentary maneuvering, flip-flopping, and brinksmanship, lawmakers gave the first round of approval necessary for the amendment to appear on the ballot in 2008. The measure still needs the endorsement of the next legislative session.

If the amendment makes it onto the ballot and residents approve it, it will leave Massachusetts’s 8,000 existing gay marriages intact but ban any new ones.

“This is democracy in action. It’s not a vengeance campaign. It’s not a hate campaign. It’s just an opportunity for the people to vote,” Kris Mineau of the Massachusetts Family Institute, a conservative group that opposes gay marriage, said. If lawmakers had failed to act on the amendment yesterday, the measure would have died, and opponents of gay marriage who collected 170,000 signatures to try to put the issue on the ballot would have had to start over again.

The pressure on lawmakers came from all sides: Gay-rights activists and the Democratic governor-elect, Deval Patrick, called on the legislature to let the measure die without a vote. Gay-rights opponents — and Massachusetts’s highest court — demanded an up-or-down vote.

The state Supreme Judicial Court — the same court that ruled in 2003 that gays have a constitutional right to marry — declared last week that lawmakers had shirked their constitutional duties by refusing to vote on an amendment submitted by the people.