A vocal group of critics, meanwhile, argues that the Fed has already done far too much, amassing a portfolio of more than $2 trillion that may impede the central bank’s ability to raise interest rates to curb inflation. Some of these critics view the rising price of oil and other commodities as harbingers of broader price increases.

“I wasn’t a big fan of it in the first place,” said Charles I. Plosser, president of the Federal Reserve Bank of Philadelphia and one of the 10 members of the Fed’s policy-making board. “I didn’t think it was going to have much of an impact, and it complicated the exit strategy. And what we’ve seen has not changed my mind.”

The Fed’s decision to buy bonds, known as quantitative easing, emulated Japan’s central bank, which started buying bonds in 2001 to break a deflationary cycle.

The American version worked well at first. From November 2008 to March 2010, the Fed bought more than $1.7 trillion in mortgage and Treasury bonds, holding down mortgage rates and reducing borrowing costs for well-regarded companies by about half a percentage point, according to several studies. That is an annual savings of $5 million on every $1 billion borrowed.

As the economy sputtered last summer, Mr. Bernanke indicated in an August speech that the Fed would start a second round of quantitative easing, soon nicknamed QE 2. The initial response was the same: Asset prices rose, interest rates fell, and the dollar declined in value.

But in addition to being smaller, and solely focused on Treasuries, there also was a problem of diminishing returns. The first round of purchases reduced the cost of borrowing by persuading skittish investors to accept lower risk premiums. With markets closer to normalcy, Mr. Bernanke warned in his August speech that it was not clear that the Fed would have comparable success in persuading investors to accept even lower rates of return.

“Such purchases seem likely to have their largest effects during periods of economic and financial stress,” he said.