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 â¿¿ 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 borrowing, 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 over 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.