Janet Yellen, President Barack Obama's nominee to lead the U.S. Federal Reserve, is sworn in to testify at her U.S. Senate Banking Committee confirmation hearing in Washington in this file photo taken November 14, 2013. REUTERS/Jason Reed

By Thomas Ferraro and Richard Cowan

WASHINGTON (Reuters) - Janet Yellen, a key force behind the Federal Reserve's unprecedented and controversial efforts to boost the U.S. economy, was confirmed by the Senate on Monday to lead the central bank just as it begins to unwind that stimulus.

When she succeeds Ben Bernanke, whose second four-year term as Fed chairman expires on January 31, Yellen will become the first woman to run the Fed in its 100-year history and just one of a handful of women heading central banks globally. She is currently the Fed's vice chair.

The vote to approve her was 56-26. She won resounding support from Democrats, who were joined by 11 Republican senators. All of the no votes came from Republicans -- a sign of discomfort with the U.S. central bank's unconventional policies as well as the partisan rancor in the Senate against any of President Barack Obama's nominees.

"With the bipartisan confirmation of Janet Yellen as the next Chair of the Federal Reserve, the American people will have a fierce champion who understands that the ultimate goal of economic and financial policymaking is to improve the lives, jobs and standard of living of American workers and their families," Obama said in a statement.

The Fed cut overnight interest rates to near zero in late 2008 as the country struggled with a deep recession that left millions of Americans out of work after the financial system imploded. It has quadrupled its balance sheet to more than $4 trillion through a series of massive bond purchase programs meant to push down longer-term borrowing costs.

Yellen, 67, spent years defending those efforts, arguing both as Bernanke's deputy and before that as head of the San Francisco Federal Reserve Bank that they would reduce borrowing costs and spur hiring and economic growth.

Now those policies appear to be working: the U.S. unemployment rate fell in November to a five-year low of 7 percent and the economy grew in the third quarter of 2013 at its fastest pace in almost two years.

Yellen's main task in the world's most powerful financial post likely will be to navigate the central bank's way out of its extraordinary stimulus, beginning with dialing down its bond-buying program.

In December, Bernanke began the process, leading the central bank to its landmark decision to shave the bond purchases to $75 billion this month from a previous monthly pace of $85 billion.

The entire program, known as QE3 because it is the Fed's third such effort at so-called quantitative easing, will likely be shuttered by late 2014 so long as the economic recovery proceeds as forecast, Bernanke said.

Many Republicans and several of Yellen's own Fed colleagues see the wind-down of that program as long overdue and warn that the buildup of bonds on the Fed's balance sheet could stoke inflation or asset-price bubbles.

The vote count was unusually low because several senators were delayed by bad weather as they attempted to return to Washington. Even so, Yellen's margin of victory was historically weak. It compares to Bernanke's weak 2010 re-appointment vote of 70-30, which at the time was the weakest endorsement on record.

"This expansionary monetary policy cannot continue into perpetuity without causing real and lasting damage to our economy," said Senator Charles Grassley, an Iowa Republican.

Those concerns notwithstanding, analysts by and large expect Yellen to stick with the "dovish" approach to policy that she has long been known for, with a focus on reducing unemployment, particularly if inflation continues to run well below the Fed's 2 percent target.

"The transition from the Bernanke Fed to the Yellen Fed likely will be a very smooth one," said Dana Saporta, an economist with Credit Suisse in New York. "While we suspect she may tolerate modestly above-target inflation to promote greater employment gains, Yellen has not wavered in her public support for the Fed's 2 percent inflation target."

Yellen has long argued that the Fed should tolerate slightly higher inflation if that is the cost of fighting high unemployment. But she has also advocated interest rate increases when she felt the threat of inflation called for them.

"She is a strong person but she is also one who is open to other people's opinion and learns quickly," said Bill Rhodes, president of William R. Rhodes Global Advisors and a former senior vice chairman at Citigroup. "I think there has been a tendency to underrate her ability, frankly."

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