WASHINGTON — Banks are not the only ones struggling in the growing financial crisis. The fund established to insure their deposits is also feeling the pinch, and the taxpayer may be the lender of last resort.

The Federal Deposit Insurance Corp., whose insurance fund has slipped below the minimum target level set by Congress, could be forced to tap tax dollars through a Treasury Department loan if Washington Mutual Inc., the nation's largest thrift, or another struggling rival fails, economists and industry analysts said yesterday.

The Treasury Department has already come to the rescue of several corporate victims of the housing and credit crunches. The government took over mortgage finance companies Fannie Mae and Freddie Mac, and helped finance the sale of investment bank Bear Stearns to J.P. Morgan Chase & Co.

Eleven federally insured banks and thrifts have failed this year, including Pasadena, Calif.-based IndyMac Bank, by far the largest shut down by regulators.

Additional failures of large banks or savings and loans companies seem likely, and that could overwhelm the FDIC's insurance fund, said Brian Bethune, U.S. economist at consulting firm Global Insight.

"We've got a ... retail bank run forming in this country," said Christopher Whalen, senior vice president and managing director of Institutional Risk Analytics.

Treasury Secretary Henry Paulson said Monday that the country's commercial banking system "is safe and sound" and that "the American people can be very, very confident about their accounts in our banking system." FDIC officials also have said 98 percent of U.S. banks still meet regulators' standards for adequate capital.

But fear is growing on Main Street as well as Wall Street about the likelihood of multiple bank failures and the strain that would put on the FDIC.

The fund, which is marking its 75th anniversary this year, is at $45.2 billion — the lowest level since 2003. At the same time, the number of troubled banks is at a five-year high.

FDIC chairwoman Sheila Bair has not ruled out the possibility of going to the Treasury for a short-term loan at some point. But she has said she does not expect the FDIC to take the more drastic action of using a separate $30 billion credit line with Treasury — something that has never been done.

The FDIC's fund is currently below the minimum set by Congress in a 2006 law. The failure of IndyMac Bank in July cost $8.9 billion.

Bair plans to propose raising the premiums that banks and thrifts pay into the fund. That plan is likely to be approved by the FDIC board, which consists of her, Comptroller of the Currency John Dugan, thrift supervision director John Reich and two other officials.