Having only a target date of mid-2015 for any increase in interest rates "sounded gloomy," as if the economy would remain weak until then, Gagnon said. Specifying an unemployment rate -- one close to a normal rate of 6 percent or less -- makes clear that the Fed will keep supporting the economy even after the job market has strengthened significantly.
"This is trying to get away from that sense of 'Oh, my God, this is all about gloom and doom,' " Gagnon said.
The Fed's new plan to link any rate increase to specific levels of unemployment and inflation mirrors a proposal pushed by Charles Evans, president of the Federal Reserve Bank of Chicago.
Updated forecasts that the Fed released Wednesday illustrate why it thinks it should continue helping the economy. It expects unemployment to remain at least 7.4 percent next year and 6.8 percent by the end of 2014. The earliest it sees unemployment dropping below 6.5 percent is the end of 2015.
It predicts the economy will grow no more than 3 percent next year before picking up to as much as 3.5 percent growth in 2014 and as much as 3.7 percent in 2015.
The Fed said it can pursue the aggressive stimulus programs because inflation remains below its target of roughly 2 percent annually over the long run. The statement said officials think the Fed can keep its benchmark short-term rate near zero as long as its one- to two-year inflation outlook doesn't exceed 2.5 percent.
The statement was approved 11-1. Jeffrey Lacker, president of Federal Reserve Bank of Richmond, objected for the eighth straight time this year. Lacker has said he thinks the job market is being slowed by factors beyond the Fed's control. And he says further bond purchases risk worsening future inflation.
The latest bond-buying program would replace an expiring program called Operation Twist. With Twist, the Fed sold $45 billion a month in short-term Treasurys and used the proceeds to buy the same amount in longer-term Treasurys.
Twist didn't expand the Fed's investment portfolio, it just reshuffled the holdings. But the Fed has run out of short-term securities to sell. So to maintain its pace of long-term Treasury purchases and to keep long-term rates low, it must spend more and increase its portfolio.
The Fed's portfolio totals nearly $2.9 trillion -- more than three times its size before the 2008 financial crisis.
The Fed has launched three rounds of bond purchases since the financial crisis hit. In announcing a third program in September, the Fed said it would keep buying mortgage bonds until the job market improved substantially.
Skeptics note that rates on mortgages and many other loans are already at or near all-time lows. So any further declines in rates engineered by the Fed might offer little economic benefit.
But besides seeking to spur lending, the Fed's drive to cut rates has another goal: to induce investors to shift money out of low-yielding bonds and into stocks, which could lift stock prices. Stock gains boost wealth and typically lead individuals and businesses to spend and invest more. The economy would benefit.
Inside and outside the Fed, a debate has raged over whether the Fed's actions have helped support the economy during the past four years, whether they will ignite inflation later and whether they should be extended. But within the Fed, so far this year only Lacker has publicly dissented from the Fed's aggressive actions to aid the economy.AP Economics Writers Paul Wiseman, Christopher S. Rugaber and Marcy Gordon contributed to this report.