Supporters of gay rights tried to block the proposal by arguing in a lawsuit that the state constitution bars any citizen-initiated amendment that seeks to reverse a judicial ruling.

The Supreme Judicial Court rejected that argument, ruling unanimously that the measure is not a reversal of its earlier ruling legalizing same-sex marriage because it would leave intact the marriages of same-sex couples who already had wed.

The state legislature will now take up the question in a constitutional convention Wednesday. Two consecutive legislative sessions have to approve the proposal, which would restrict marriage to between one man and one woman, before it ...