Thompson Gets Key Endorsement From Bronx Borough President In another sign of his deep ties to established leaders in minority communities, William C. Thompson Jr. collected a key endorsement in his bid for mayor Tuesday from Ruben Diaz Jr., the Bronx borough president. Mr. Diaz's endorsement was not much of a surprise, given that he had been a vocal supporter of Mr. Thompson, a former comptroller, in his unsuccessful bid in 2009 to unseat Mayor Michael R. Bloomberg. But his endorsement had been coveted by all the major Democratic candidates, who know that Latinos could make up 20 to 25 percent of the Democratic electorate in September. In a news conference at City Hall, Mr. Diaz repeatedly cited Mr. Thompson's "character," "résumé" and - in what was viewed as a subtle dig at Christine C. Quinn, the City Council speaker - "temperament." And while he said that he had been wooed by other Democrats - Ms. Quinn; Bill de Blasio, the public advocate; and John C. Liu, the comptroller - he suggested that he didn't agonize over his choice because of Mr. Thompson's reputation as a "coalition builder" for the past two decades. "I slept with ease last night," Mr. Diaz said. The endorsement comes as Mr. Thompson steadily rolls out endorsements and hires staff members, despite middling public polling numbers and lingering doubts about the energy level of his campaign. On Monday, Mr. Thompson picked up the backing of two state assemblymen: Karim Camara of Brooklyn, who is the head of Albany's black, Hispanic and Puerto Rican caucus, and Herman D. Farrell Jr., a former head of the Manhattan Democratic Party. He also announced the hiring of several staff members, including Hank Sheinkopf, a prominent Democratic consultant who worked for Mr. Bloomberg in 2009, and two people who worked previously for another likely mayoral contender, former Representative Anthony D. Weiner. Last week, Mr. Thompson secured the support of two Bronx Latino leaders, Representative Jose E. Serrano and his son, State Senator Jose M. Serrano.