"I look forward to working side-by-side with Alex to bring about the change Sacramento needs," Newsom said. "His background, his experience and his ideas will be an incredible asset to this campaign."
Newsom, mayor of San Francisco, is expected to face California Attorney General Jerry Brown in next June's Democratic gubernatorial primary. Los Angeles Mayor Antonio Villaraigosa had been weighing a bid for the office, but announced recently that he would not enter the race.
Padilla previously served on the Los Angeles City Council and was the first Latino to ever serve as council president.
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"While his (Newsom's) leadership is well known throughout Northern California, I am confident that, as people get to know him throughout the southern parts of our state, they will see him as an extraordinary leader with an uncommon capacity to inspire and unite people," Padilla said.