BOSTON — Massachusetts lawmakers on Thursday blocked a proposed constitutional amendment banning gay marriage from reaching voters, a stunning victory for gay marriage advocates and a devastating blow to efforts to reverse a historic 2003 court ruling legalizing same-sex marriage.

The narrow 45-151 vote means Massachusetts remains the only state in the nation to allow same-sex couples to marry. The question needed the approval of 50 of 200 senators and representatives to advance to the 2008 ballot.

Opponents of gay marriage vowed to press on, but Thursday's defeat — after more than three years of sometimes wrenching debate — could prove insurmountable.

Any effort to mount a new ballot question would take years at a time when political support in Massachusetts is swinging firmly behind gay marriage and public attention has moved onto other issues, including the state's landmark health care law.

For gay couples, the vote marked what could be the end of a struggle that began in 2001, when seven same-sex couples, denied marriage licenses, sued in Suffolk Superior Court in Boston to challenge the state's gay marriage ban.

More than 8,500 same sex couples have married in Massachusetts since they became legal in May 2004.

The vote is also a victory for the state's Democratic leadership, including Gov. Deval Patrick, a vocal supporter of gay marriage, who pressed lawmakers up until the final moments to block the measure.