Two U.S. Senate Democrats on Friday said they would vote against Ben Bernanke for a second term as Federal Reserve chairman, adding an element of uncertainty for Wall Street.

The White House said President Barack Obama remained confident the Democratic-controlled Senate would muster the 60 votes needed to clear any procedural hurdles and confirm Bernanke's nomination, but doubts weighed on Wall Street.

Senators Russ Feingold and Barbara Boxer announced their opposition, bringing the known 'no' votes among Democrats to four. Several Republicans and one independent have also said they will vote against the nomination.

A number of senators have sought to block the nomination with 'holds,' forcing Senate leaders to secure a super-majority of 60 votes in the 100-member chamber to move the nomination.

With the U.S. job market in disarray and voters angry at Wall Street firms for benefiting from government and Fed-orchestrated bailouts, the U.S. central bank and its chairman have become targets for discontent.

Bernanke, a Republican first named to the Fed as chairman by former President George W. Bush, was renominated by Obama to the post last year. His current term expires on January 31.

Senate Democratic aides and a senior Republican leadership aide said they still expect Bernanke to be confirmed for another four-year term, but added it was yet unclear what the vote will be and that both parties are taking head counts.

Democrats are nervous, the senior Republican aide said. But I don't think Democrats are going to kill the president's nominee. I think he will be confirmed.

He said if Democrats are unable to secure the 60 votes needed to clear procedural hurdles they will probably not even bring the nomination up for a vote.

A Democratic aide said: We don't know where all the votes are, but I anticipate that the votes will be there to confirm.

Feingold echoed some of the same concerns cited by Republican lawmakers who have said they will oppose Bernanke.

Under the watch of Ben Bernanke, the Federal Reserve permitted grossly irresponsible financial activities that led to the worst financial crisis since the Great Depression, Feingold said in a statement.

Senator Bernie Sanders, an independent and a leading Bernanke foe, thinks it's close on whether the votes will be there for confirmation, an aide said.