BOSTON (AP) — As gay advocates prepared for the first-ever Statehouse hearing on legalizing same-sex marriages, key lobbyists said that legislative support could be building for a bill that would do the exact opposite.

The House and Senate are scheduled to meet in joint session Nov. 12 to consider a constitutional amendment that would define marriage as a union between one man and one woman — an initiative that was defeated last year by more than half of the state’s 200 lawmakers.

But an impending decision by the state’s highest court, which many believe could set a national precedent on the legalization of gay marriage, may be shaking up the Legislature, according to Arline Isaacson of the Massachusetts Gay and Lesbian Political Caucus.

“Legislators are very nervous about this because of the (Supreme Judicial Court) decision,” Isaacson said. “It could change everything. It makes it less abstract.”

The Judiciary Committee is holding a public hearing on legislative initiatives to legalize gay marriage and civil union, as well as several other hot-button social issues, including abortion restrictions, assisted suicide and breast-feeding in public.

It is the first time the Legislature has ever heard a bill that would legalize gay marriage. The Legislature has debated the issue for years, but as part of bills that would have banned gay marriage.

“We will no longer be denied the most basic benefits that marriage provides,” said Stacy Roth, of the Lesbian, Gay, Bisexual, Transgender Political Alliance of Western Massachusetts. “Civil marriage is a civil right.”

Since July 2002, when the prior constitutional amendment was rejected by the Legislature, the landscape has changed nationally and internationally, as well as within the state.

Canadian courts legalized gay marriages, the U.S. Supreme Court extended privacy rights to gay couples in an anti-sodomy case, and the Massachusetts high court heard testimony on a case that could make the state the first in the country to issue marriage licenses to gay couples.

A decision in that case, which was filed by seven same-sex couples who were denied licenses, has been expected since early July. The court, however, waived its own internal deadline, leaving advocates on both sides unclear how to approach the impending legislative debates.

“Everybody is holding their breath and waiting to see,” said Rep. Alice Wolf (D-Cambridge), a longtime gay rights advocate.

Speculation that the court may be leaning toward legalization of gay marriage has transformed the debate about the constitutional amendment from the abstract to the concrete. This has weakened the opposition of people who previously said they opposed the amendment because it addressed an unlikely threat, according to Dr. Ron Crews of the Massachusetts Family Institute.

“That has removed the argument from some legislators,” Crews said. “We need more support than we did last year and I think we have it.”

Last year, because the amendment was sparked by a citizen petition, it only required approval from more than 25 percent of the Legislature — or at least 51 lawmakers. This year, because the amendment was proposed by a legislator, it would require approval by at least half of the Legislature.

If approved on Nov. 12, the amendment would then have to be approved again during the 2005-2006 session before appearing on the statewide ballot in November 2006. Another option would be to delay a vote on the amendment until next year.

The Legislature’s leading gay rights advocates are certain the opposition remains strong and that passage is unlikely, especially given the higher threshold.

“Opponents are trying to use the pending SJC case as a scare tactic,” said Sen. Cheryl Jacques (D-Needham). “But most lawmakers recognize this for what it is, which is a discriminatory measure.”