President Barack Obama's chief of staff, Rahm Emanuel, and Treasury Secretary Timothy Geithner were on the telephone throughout the day to crucial senators trying to shore up support, said two senior administration officials, speaking on condition of anonymity to more freely discuss behind-the-scenes activity.

Four Democrats say they will vote against Bernanke on the Senate floor. At least two senators who voted for Bernanke in the Senate Banking Committee last month are reconsidering their support.

Still, the administration's worries about the status of the nomination also lessened somewhat, despite the Democratic defections, by the knowledge that several Republicans were committed for Bernanke, one official said.

Many others have not made their inclinations known, suggesting a vacillation in the Senate over Bernanke and his stewardship of Wall Street both before and after the financial crisis.

While no one is declaring his confirmation doomed, the emergence of opposition and the shift by some to undecided illustrate just how difficult the terrain has gotten for Obama, especially since a Republican Senate victory in Massachusetts this week.

The roster arrayed against him grew Friday, with Democratic Sens. Barbara Boxer and Russ Feingold announcing their opposition.

"It is time for a change - it is time for Main Street to have a champion at the Fed," Boxer said.

Sen. Jeff Merkley, a Democrat who opposed Bernanke in committee, also said Friday he was opposing Bernanke's nomination. He blamed Bernanke not only for missing signs of the smoldering crisis. "Ben Bernanke helped set the fire," Merkley said on Friday.

Democrat Byron Dorgan announced Thursday he would oppose Bernanke on the Senate floor.

White House deputy press secretary Bill Burton, talking to reporters as Obama headed to Ohio Friday, said the president has "a great deal of confidence" in the actions Bernanke already has taken and believes he's "the best person for the job."

Burton said the White House still believes that Bernanke, 56, will get enough votes in the Senate to run the central bank for another four years.

Bernanke faces a 60-vote Senate hurdle because Sen. Bernie Sanders, an independent liberal, has placed a "hold" on the nomination, meaning it will require a super-majority to confirm him in the 100-member Senate.

The political waters have been getting rougher for Bernanke, and the Senate confirmation vote may be much tighter than many had anticipated just a month ago. Counting votes against him in committee, Bernanke has at least 10 senators publicly against him.

Bernanke has no real Senate constituency with either party because he was appointed to his first term by President George W. Bush but is now closely linked to Obama's economic policies.

A time for a vote still hasn't been set. Officials at one time had hoped that it would come this week. Bernanke's term expires on Jan. 31.

If Bernanke is not confirmed before his term expires, Fed Vice Chairman Donald Kohn would probably step in as chairman and run the central bank on a temporary basis.