WASHINGTON — Jack Lew, President Barack Obama's nominee for treasury secretary, is a premier federal budget expert who would take the helm of the government's main agency for economic and fiscal policy just as the administration girds itself for a new confrontation with congressional Republicans over the nation's debt and deficits.

Obama will nominate Lew on Thursday afternoon, continuing to put a second-term imprint on his Cabinet by choosing yet another close ally to a key government post.

A year ago, almost to the day, Obama appointed Lew as his chief of staff, taking him from his perch as director of the Office of Management and Budget into the White House's tight inner circle.