WASHINGTON - US President Barack Obama will nominate Fed number two Janet Yellen on Wednesday to run the world's most influential central bank, providing some relief to markets that would expect her to tread carefully in winding down economic stimulus.Yellen, a Jewish economist, studied economics at Yale University and taught at Berkeley for more than a decade before her first stint as a Fed board governor from 1994 to 1997, a post she left to head President Bill Clinton's Council of Economic Advisers.Respected economistYellen has enjoyed strong support from Democrats. In an unusual move, 20 Senate Democrats signed a letter earlier this year pressing Obama to turn to the former professor from the University of California at Berkeley.Her Republican backing is much softer. Many Republicans worry Fed policy of holding overnight interest rates at zero and buying bonds aggressively to drive other borrowing costs lower could lead to asset bubbles and an unwanted pickup in inflation."I voted against Vice Chairman Yellen's original nomination to the Fed in 2010 because of her dovish views on monetary policy," Senator Bob Corker of Tennessee said in a statement. "We will closely examine her record since that time, but I am not aware of anything that demonstrates her views have changed."Senator Richard Shelby of Alabama, another Republican, said he has concerns about her "proclivity to print money" and her record as a bank regulator.Still, Yellen is expected to garner enough support to secure the 60 votes needed to overcome any procedural hurdles in the 100-seat Senate. Democrats control the chamber 54-46.A respected economist whose research has taken her deep into theories of monetary policy, Yellen has earned a reputation as one of the Fed officials most worried about unemployment and least concerned about inflation."With employment so far from its maximum level and with inflation running below the committee's 2 percent objective, I believe it's appropriate for progress in the labor market to take center stage in the conduct of monetary policy," she said in March.She later served as president of the San Francisco Federal Reserve Bank, where her first-hand view of the overheated real estate market helped her see the dangers of the housing bubble earlier than many of her colleagues.As Fed chair, Yellen would arguably be the most powerful woman in the world.She has been central to moving the Fed toward more clarity and precision in its communications, an openness which she sees as the key to an effective monetary policy.Yellen led a panel of officials who rewrote the Fed's rules on communications and helped convince her colleagues to adopt an explicit inflation target for the first time last year.Her selection bolsters the credibility of promises the Fed has made about the future course of monetary policy that have been a hallmark of its approach ever since it dropped interest rates to zero in 2008.Specifically, she could be expected to abide by, if not strengthen, the Fed's stated commitment to keep rates steady at least until the US jobless rate hits 6.5 percent, as long as inflation does not threaten to pierce 2.5 percent. The nation's jobless rate stood at 7.3 percent in August.Easy moneyYellen, who has long argued that the Fed should tolerate slightly higher inflation if that is the cost of fighting high unemployment, has never dissented on a Fed policy decision.But she also has not shied away from advocating rate rises if she feels the situation calls for it. In 1996, after then-Fed Chairman Alan Greenspan had repeatedly put off raising rates, she and a colleague went to him to argue that the central bank was at risk of courting inflation.Once again, the central bank is facing criticism from some quarters that it is risking inflation. Its controversial bond purchases have put the Fed on track to buy some $3 trillion in mortgage and Treasury debt.The easy money was aimed at digging the US labor market out of the deep hole caused by the 2007-2009 recession.While it pushed US borrowing costs to record lows and sent US stocks to record highs, the loose policy also fueled resentment in some emerging markets, who had to contend with a flood of hot money as investors sought higher returns.Now, the flood gates are reversing.The mere mention by Bernanke in May that the Fed could soon begin to ease up on its monthly purchases sent global financial markets reeling and U.S. borrowing costs sharply higher. Currencies and equities in many emerging markets plunged - underscoring the delicate task Yellen would face.Despite the Fed's aggressive efforts to prop up the economy, growth has been lackluster and the labor market is still sickly.The nomination will put Yellen on course to be the first woman to lead the institution in its 100-year history. The advocate for aggressive action to stimulate US economic growth through low interest rates and large-scale bond purchases would replace Ben Bernanke, whose second term as Fed chairman expires on January 31.If confirmed by the US Senate, which is expected to endorse her, she would provide continuity with the policies the Fed has established under Bernanke. Analysts say she would move cautiously in reining in policies in place to shore up the world's largest economy.Expectations that the Fed might start to taper its stimulus program have roiled financial markets since May and the central bank shocked investors in September by maintaining its cash injections of $85 billion a month in full."Thank God Yellen will be nominated under the current circumstances. You don't want a change at the central bank right now," said Dan Fuss, a portfolio manager at Loomis Sayles in Boston. "This Yellen news is one uncertainty lifted from already nervous markets."Her nomination would come during a political stalemate in Washington that has closed the US government and threatened a US default if lawmakers fail to raise the $16.7 trillion debt ceiling by an October 17 deadline.US stock index futures rose and the dollar slipped on the news of Yellen's pending nomination. The debt standoff is fueling expectations the Fed may delay any plans to reduce its stimulus for now.If confirmed, she would join the Fed's honor roll along with such household names as Paul Volcker and Alan Greenspan, predecessors as head of an institution that can influence the course of the world economy."I believe she'll be confirmed by a wide margin," said Senator Charles Schumer, a Democrat from New York.Described as a "good egg" by fellow Fed policymaker Richard Fisher and a "very able person" by Japan's Chief Cabinet Secretary Yoshihide Suga, her most immediate challenge may be to determine when the Fed should scale back its bond buying.After September's surprise decision against tapering, many economists now think the Fed will not move until Bernanke has left office.Obama turned to Yellen, 67, after his former economic adviser Lawrence Summers withdrew from consideration in the face of fierce opposition from within the president's own Democratic Party, raising questions about his chances of congressional confirmation. The contest between Summers and Yellen played out all summer in a public way not usually associated with the selection of the top US central banker.Obama is scheduled to announce his nomination at the White House at 3 p.m. EDT (1900 GMT), a White House official said on Tuesday. Bernanke is expected to attend.