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 new bond-purchase plan would join a program announced in September. Under that program, the Fed is buying $40 billion a month in mortgage bonds to try to force already record-low home-loan rates lower in a bid to encourage home buying. The total Fed bond purchases from the two programs would remain $85 billion. "A little better unemployment report for November doesn't change the Fed's view that the economy needs additional help," said Diane Swonk, chief economist at Mesirow Financial. If the Fed decides not to replace Twist with a new bond-buying program, the monthly amount of its long-term Treasury purchases would decline by half. Long-term borrowing rates might rise as a result. When the Fed pumps more money into the financial system and adds to its portfolio, it's called quantitative easing, or QE. Critics argue that QE risks fueling inflation later. The Fed's portfolio totals nearly $2.9 trillion â¿¿ more than three times its size before the 2008 financial crisis. The Fed has launched three rounds of QE since the financial crisis hit. In announcing QE3 in September, the Fed said it would keep buying mortgage bonds until the job market improved substantially. It also extended its plan to keep its benchmark short-term rate near zero through at least mid-2015. And it raised the possibility of taking other steps. 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. Inside and outside the Fed, a debate has raged over whether the Fed's actions have helped support the economy over the past four years, whether they will ignite inflation later and whether they should be extended.