WASHINGTON — The Federal Reserve announced an unconventional plan on Wednesday to reduce borrowing costs for businesses and consumers, trying once more to spur economic growth despite urgings from Republicans that it refrain from any expansion of its stimulus program.

The Fed said that it would invest $400 billion in long-term Treasury securities over the next nine months, using money raised by selling its holdings of short-term federal debt, in an attempt to drive down interest rates on mortgage loans, corporate bonds and other forms of credit.

The Fed’s policy-making committee said in a statement that it was acting because it saw little prospect that the economy would expand fast enough and soon enough to help the 25 million Americans unable to find full-time work. It also said there was a significant risk that “strains in global financial markets” could further damage prospects for recovery.

“Growth remains slow. Recent indicators point to continuing weakness in overall labor market conditions and the unemployment rate remains elevated,” the Fed said in the statement, which listed its reasons for worrying about the anemic condition of the American economy. “Household spending has been increasing at only a modest pace in recent months.”