WASHINGTON -- Jack Lew, President Barack Obama's likely 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 is expected to nominate Lew as early as Thursday, continuing to put a second-term imprint on his Cabinet by choosing yet another close ally for 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. In selecting Lew to replace Treasury Secretary Timothy Geithner, Obama not only would be picking an insider steeped in budget matters but also a tough bargainer. Some Republicans complain that Lew has been unyielding in past fiscal negotiations. If confirmed, Lew would assume the post in time for the administration to tangle anew with Republicans over a confluence of three looming fiscal deadlines -- raising the $16.4 trillion federal borrowing limit, averting automatic spending cuts to defense and domestic programs, and renewing a congressional resolution that has been keeping the government operating. Those three events, if unresolved, would have a far greater negative effect on the economy than the "fiscal cliff" Obama and Congress avoided a week ago. Lew, 57, has often been described as a "pragmatic liberal" who understands what it takes to make a deal even as he stands by his ideological views. "He's a political guy. He didn't get where he is today by being a shrinking violet," said Paul Light, a public policy professor at New York University and an acquaintance of Lew's. "But he's really a doer. He's the kind of guy you want at the table if you want to get something done." One senior Republican senator, Alabama's Jeff Sessions, voiced opposition to Lew. Though Lew may face a tough confirmation in the Senate, he's not likely to encounter the type of stiff opposition that is already mounting against former Sen. Chuck Hagel, a Nebraska Republican whom Obama has tapped to be his defense secretary.