SAN ANTONIO — At a sunny pavilion amid a rainbow of ballet folklorico dancers and mariachi music, Hillary Rodham Clinton tried her best to speak Spanish as she picked up a major endorsement and sought to transfer the momentum off her performance in the first Democratic debate on Tuesday to the critical constituency of Latinos.

Julián Castro, the former mayor of this South Texas city and currently the secretary of housing and urban development, endorsed Mrs. Clinton at a Latinos for Hillary organizing event here on Thursday, the first of a series of such events designed to drum up enthusiasm among Latinos beyond the early four voting states.

“She has always, always, been there for us,” Mr. Castro told the crowd as people waved Hillary signs that read “Estoy Contigo!” (“I’m with you!”)

Switching to Spanish, Mr. Castro said “la diferencia” between Mrs. Clinton and the Republicans is that “she respects the Latino community.”