WASHINGTON -- The Federal Reserve ramped up its effort to revive the economy, declaring it would buy as much as $300 billion of long-term U.S. Treasury securities in the next few months and hundreds of billions of dollars more in mortgage-backed securities.

The Fed had already cut its benchmark interest-rate target to near zero. Unable to go lower, the central bank now is essentially printing money to raise the supply of credit and thus push down the longer-term rates paid by families and companies on mortgages and other key...