WASHINGTON — President Obama announced Thursday that he was nominating his chief of staff, Jacob L. Lew, to be his new Treasury secretary, assigning one of the top Cabinet roles to a longtime budget chief and former adviser to the late House Speaker Thomas P. “Tip” O’Neill Jr.

Lew, who would replace Treasury Secretary Timothy Geithner, has served a number of roles in government over his three-decade career, including twice serving as director of the Office of Management and Budget.