Lew has urged Congress to raise the current $16.7 trillion borrowing limit before then. President Barack Obama has repeatedly said he won't negotiate with Republicans over raising the debt limit as he did during a previous such fight in August 2011.
The 2011 standoff was resolved at the last minute. The borrowing limit was raised before the government defaulted on its debt obligations. But the prolonged political impasse led Standard & Poor's to downgrade long-term U.S. debt for the first time in history.
Most analysts also expect the current debt-limit standoff to be resolved before the deadline given the economic crisis that would ensue if it isn't.
But Mark Zandi, chief economist at Moody's Analytics, said it could take the threat of further turbulence in financial markets to break the logjam.
"I think the most likely scenario is that policymakers only act when financial markets begin to sell off," Zandi said. "It will require lower stock prices, a lower dollar and turmoil in the bond market to get Washington to move."
Zandi is among those who think market turmoil will delay any pullback in the Fed's bond purchases. He foresees no slowdown in the purchases until 2014.
"With each day that the lawmakers can't get it together, the economic damage mounts, and the day the Fed starts tapering is pushed off," he said.
It's always possible that Congress could act faster than some expect to resolve the shutdown and the government's borrowing limit. If so, some say the Fed might still act before year's end to scale back its stimulus.
But Vincent Reinhart, chief economist at Morgan Stanley and a former top economist at the Fed, said he saw the likelihood of a cut in bond purchases as around 5 percent at the Fed's October meeting and 10 percent at the December meeting.
Sohn said the Fed's meeting in March could be the first one when it might feel the economy was strong enough to withstand a pullback in bond purchases.
If the slowdown in bond buying began in March, it would be a full six months after the September date where many had initially expected the Fed's pullback to begin.
That's why Sohn thinks the mid-2014 point that Bernanke had mentioned as a likely time for the bond purchases to end will likely slip until the end of 2014.
Sohn said he expects the Fed to eventually take small steps in cutting its $85 billion in monthly purchases, possibly in reductions of $10 billion to $20 billion that would end in December 2014.
"The Fed wants to take measured steps, and I don't think they will move until Yellen feels she is in firm command," Sohn said. "Washington's budget craziness has made the Fed more cautious."
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