NEW YORK — President-elect Barack Obama has chosen New Mexico Gov. Bill Richardson to be Commerce secretary, adding a prominent Hispanic and onetime Democratic rival to his expanding Cabinet.

Obama planned to announce the nomination after Thanksgiving, according to a Democratic official familiar with the discussions. The official was not authorized to speak publicly about the negotiations and did so on condition of anonymity.

Richardson, 61, had a distinguished and visible career in Washington before returning to New Mexico, where he was elected governor in 2002. Richardson served as U.N. ambassador under President Bill Clinton and later as energy secretary. He was in the House from 1983 to 1997.

Clinton sent Richardson on several high-level diplomatic missions while he was in Congress, including direct talks with Iraqi leader Saddam Hussein.

Richardson is a seasoned international negotiator who mediated with North Korea over the downing of two U.S. Army helicopter pilots; hammered out a deal with then Iraqi President Saddam Hussein for the release of two U.S. oil workers; won the release of three Red Cross workers held captive by Sudanese rebels; and was later sought out by the North Koreans to discuss nuclear issues.

Richardson ran for the Democratic presidential nomination but dropped out in January after a poor showing in early contests. He went on to endorse Obama at the height of the Illinois senator’s primary contest with Sen. Hillary Rodham Clinton, angering many Clinton’s supporters who viewed the endorsement as a disloyal snub.

His success, said David Goldwyn, national security deputy at the United Nations in the late 1990s, stems from the many different facets of his personality — whether it’s the athlete (he bonded with Fidel Castro over baseball), the tireless adversary (he spent four hours at the table trying to persuade President Laurent Kabila to let a U.N. team investigate massacres in Congo) or the regular guy.

“Richardson is all about crashing through boundaries,” Goldwyn said in 2007. “He says hello to the security guy, and if he’s Hispanic he’ll say something in Spanish. If he’s African-American, he might call him ‘his brother.’

“People think it’s undisciplined and so they think he’s undisciplined, and that is a mistake. … His personality gets him in the door. From there, he’s got to deliver the message, he’s got to be persuasive, and he’s got to secure the objective. That’s where the other part of his personality comes in: his relentlessness.”