WASHINGTON -- The Federal Reserve said Wednesday it will shuffle $400 billion of its portfolio to try to drive down long-term interest rates and get the economy going. But economists doubted it would do much good, the stock market sold off, and the Fed itself was unusually divided over the strategy.

The idea is to make mortgages and other major loans cheaper and encourage people and businesses to spend more money -- providing a lift to the broader economy, which has slowed sharply more than two years after the recession.

But economists pointed out that Americans, still feeling insecure about the future and inclined to save rather than borrow, might not be willing to take on more debt, even at lower rates, or eligible to get it. Others see no reason to jump into the housing market when prices are still falling.

"Frankly, I don't see it having any meaningful impact on the economy," said Bernard Baumohl, chief global economist with the Economic Outlook Group. "What the Fed did today was a distraction."

Yields on U.S. government debt were already among the lowest on record, and investors drove them down further after the Fed announcement. The yield on the 10-year Treasury bond, an indicator for mortgages and other long-term loans, closed at 1.86 percent, down from 1.93 percent the day before and the lowest since at least 1962.

Along with the strategy statement, the Fed gave a stormy overview of the economy -- slow growth, high unemployment and a slumping housing market. The Fed has already said it will keep short-term interest rates super-low into 2013, a sign that the central bank was not optimistic.

Three members of the Federal Open Market Committee, the policymaking arm of the Fed, dissented. There are 10 members, including Chairman Ben Bernanke, and usually no more than two dissent. The three have said the Fed's policies may be raising the risk of inflation.

The stock market fell quickly after the Fed's 2:30 p.m. announcement.

Wall Street had expected the Fed move for weeks.

"It's being viewed as perhaps an admission that this is a longer-term issue that the U.S. economy is facing and not one that's going to be solved over a couple of years," said Oliver Pursche, president of Gary Goldberg Financial Services.

The Fed will sell $400 billion from its holdings of short-term U.S. government debt -- Treasury bills and notes that mature in three years or sooner. It will use that money to buy Treasury notes and bonds with maturities of six to 30 years. The Fed said the shift would be complete by June.

By comparison, the Fed spent about $2 trillion on two rounds of bond-buying designed to lower long-term interest rates. The Fed's total portfolio is $2.9 trillion, almost all of it in Treasury securities and mortgage-backed securities.