An announcement by new Senate President Therese Murray that she will allow a vote on a ban on gay marriage, is far from a guarantee that the proposed constitution amendment will pass, local legislators said Friday.

The lawmakers said they are pleased Murray will not use parliamentary procedures to block a vote, even though she's an opponent of the ban.

But, they said months of "hardball" politics stand in the way of passage.

The measure is an initiative petition brought forth by a citizens group, so it only needs the approval of 25 percent of legislators in two consecutive constitutional conventions.

It would then go before voters in a referendum. The amendment passed it first hurdle in January when it won narrow approval. Another affirmative vote would be needed this year to get it on the 2008 ballot.

Twenty-five percent would seem to be a low threshold to get over, but supporters of the ban said backing for the proposal has been slipping.

There are 200 members of the state Senate and House combined. The measure got 62 votes in January, 12 more than needed.

Since that time, several supporters have left office and have been replaced by opponents of the ban.

State Rep. Richard Ross, R-Wrentham, said he was glad that Murray would allow a vote. As Senate president, Murray would be the presiding officer at the constitutional convention.

Ross said Murray may have opposed a ban on gay marriage when she was a rank and file senator and wanted to adjourn the constitutional convention without voting, but as Senate president she has to take a broader view.

"I think she is doing her job as Senate president," he said.

State Rep. Betty Poirier, R-North Attleboro, predicted that legislative leaders would not hold another constitutional convention until they are sure they have enough votes to defeat the amendment.

"I'm not optimistic at all," she said. "I don't hold out a lot of hope."

She said another possibility is that if opponents fear they do not have the votes to win, they will boycott the session so there is not enough legislators present to reach a quorum, thereby cancelling the session. But, state Rep. John Lepper, R-Attleboro, said just the fact that Murray is committed to allowing a vote is a victory for the initiative process.

Previously, the Legislature has tried to avoid votes by adjourning sessions before the matter could be raised.

All that changed last year when the state Supreme Judicial Court ruled that the Legislature had an obligation to vote, he said.

Lepper said if his side loses because of "arm twisting and cajoling" by the other side, he can live with that because that is the way politics sometimes works.

"There are going to be all kinds of efforts to change people's votes. We're in the Massachusetts Legislature. Hardball politics goes on all the time," he said.

What he could not abide, he said, is "blowing up the constitutional process" by not holding a vote.

"It's definitely possible we could lose the vote, but at least we're going to have a vote. That means there is hope," he said.