An assemblywoman who leads the Brooklyn Democratic Party and who has been a key backer of Mayor Eric Adams endorsed his main rival in the upcoming mayoral election, former Gov. Andrew M. Cuomo, on Sunday.

The decision by the assemblywoman, Rodneyse Bichotte Hermelyn, is one of the clearest signs yet that the winning coalition Mr. Adams built in 2021 has been completely fractured. She is not only endorsing Mr. Cuomo but will serve as his senior political adviser, an honorary role.