As four Republican state senators, one by one, agreed to break with their party and cast a politically risky vote to legalize same-sex marriage last year, Gov. Andrew M. Cuomo, Mayor Michael R. Bloomberg and gay-rights advocates vowed to do everything in their power to protect them against political retribution.

But when the Legislature returns to Albany next month, only one of those four senators will be among those sworn into office. One, facing the prospect of a tough challenge, decided not to run again; a second was defeated by a more conservative Republican in a primary, and on Thursday, a third conceded defeat after a monthlong paper-ballot counting process in a three-way race in which a more conservative candidate drew so many votes from him that the race was won by a Democrat.

The fates of the same-sex-marriage-supporting Republicans in New York are a jarring note for same-sex marriage advocates, who last month claimed four victories on ballot measures in Maine, Maryland, Minnesota and Washington, and last week celebrated the Supreme Court’s decision to hear two cases relating to same-sex marriage.