The issue of gay marriage is back in the hands of the state's highest court, due to an effort led in part by state Senate Majority Leader Frederick E. Berry of Peabody.

Senators voted yesterday to ask the court for clarification after weeks of debate over whether Vermont-style civil unions would satisfy the Supreme Judicial Court's November ruling that gay marriage is legal in Massachusetts.

Gay marriage advocates and a law professor say they are certain the court will rule that anything less than allowing gay marriage is unconstitutional. Civil unions give same-sex couples the benefits and responsibilities of marriage without actually legalizing gay marriage.

"They've already gone through this process and determined that only marriage is going to satisfy the (state) Constitution," said Martha Davis, an associate professor at Northeastern University's School of Law. "The chance of them finding that civil unions satisfy the Constitution is pretty small."

The Senate gave preliminary approval yesterday to a bill filed by Berry and Sen. Joan Menard, D-Somerset, that would create civil unions. Lawmakers then voted to ask the court for an "advisory opinion" on whether civil unions are an acceptable alternative to gay marriage.

The court must respond to the request, but there is no time limit on when it must make a decision.

"We would like to have the court either say X or Y," Berry said, "so we'll have a clear clarification on their ruling. They didn't give us a clear direction"

Given the choice, Berry, a Democrat, said he would prefer allowing civil unions rather than letting gay couples marry.

Because the Senate votes were taken during an informal session, a single objection could have blocked the action, but no senator objected.

Sen. Jarret Barrios, D-Cambridge, who is gay and a supporter of gay marriage, favored asking the court's opinion, although he feels civil unions are a "separate but unequal system" for gay couples.

"It would be important for this body to avoid unnecessary, divisive discussions if in fact our Supreme Judicial Court will find that this misguided civil unions bill doesn't meet constitutional muster," Barrios said.

The SJC issued its decision on gay marriage on Nov. 18 and gave the Legislature 180 days to act as it "deems appropriate" before the decision takes effect.

While some lawmakers said the court's decision leaves room for a civil unions bill, supporters of gay marriage say that alternative would fall short of what the court ordered. Civil unions do not guarantee gay couples the right to federal benefits for married couples, including Social Security.

"The court has been very clear that only civil marriage licenses give us total equality," said Arline Isaacson, of the Gay and Lesbian Political Caucus. "But we realize there is some confusion among legislators."

Gov. Mitt Romney is pushing a constitutional amendment that would define marriage as a union between one man and one woman. The process would take up to three years and require a ballot vote in November 2006.