BOSTON —  Lawmakers in Massachusetts, the only state where gay marriage is legal, on Tuesday voted to advance a proposed constitutional amendment to ban gay marriage, a critical step toward putting the measure the 2008 ballot.

The proposed amendment, which would define marriage as between one man and one woman but ban future gay marriages, still needs approval of the next legislative session before it can go onto the ballot.

The vote Tuesday in the constitutional convention came without debate, immediately after Senate President Robert Travaglini officially opened the joint session.

Earlier in the day, Gov-elect Deval Patrick had met with Travaglini and House Speaker Salvatore DiMasi to urge against a vote, calling it a "question of conscience." He said the proposed amendment was the first time the amendment process was being used "to consider reinserting discrimination into the constitution."

But the state Supreme Judicial Court ruled last week that lawmakers' had shirked their constitutional duties in November by recessing instead of voting on the proposal.

The supporters of the amendment collected signatures from 170,000 people in an effort to get the question on the ballot.

The amendment would need to be approved by 50 member of the current Legislature and 50 members of the new Legislature before going to voters on the 2008 ballot. On Tuesday, 61 lawmakers backed moving the measure forward, compared to 132 opposed.