New York is within a single vote of legalizing gay marriage after a second Republican state senator said on Tuesday that he would support the measure should it come to the floor this week.

The senator, Roy J. McDonald, from the capital region, made his comments to reporters amid growing indications that Republican leaders would bring the bill to an up or down vote on Thursday or Friday.

Three other Republican state senators, speaking on condition of anonymity because their conference had not yet formally debated the measure, said they believed the bill was almost certain to come up for a vote and that it would likely pass, making New York the sixth, and largest, state in the nation to legalize same-sex marriage.