WASHINGTON — Now that the federal government has thrown a lifeline to mortgage giants Fannie Mae and Freddie Mac, taxpayers could be on the hook for billions more if the crisis of confidence spreads.

There were encouraging signs Monday for the rescue plan, but also signs of concern — notably on Wall Street, where shares of the two companies slumped further — that the plan won't be enough.

Other banks are already teetering. National City Corp. shares fell nearly 15 percent on rumors of financial trouble, even though it said it was experiencing no unusual depositor or creditor activity. And Washington Mutual Inc.'s shares fell 35 percent, to a paltry $3.23 amid worries about whether it had enough cash to handle the mortgage market downturn. WaMu said it did.

And worried customers lined up Monday to pull cash out of their accounts at IndyMac Bank, seized Friday by the federal government.

Some critics said they fear that the Fannie-Freddie rescue effort will make more bailouts inevitable by sending a message that some institutions are too big to fail and thus encouraging risky behavior.

"It sends the wrong message to the world," said Joshua Rosner, managing director of the research firm Graham, Fisher & Co. in New York.

Sung Won Sohn, an economics professor at the Smith School of Business at California State University Channel Islands, cited soaring oil costs, a weakening economy and an unstable housing market that he said will only get worse.

"I don't think these steps are enough to arrest the deterioration," he said.

As long as more homeowners default on mortgages, losses to financial institutions will mount. Those losses already exceed $400 billion, and some analysts believe that they will top $1 trillion before the housing carnage is over.

By comparison, Congress has authorized $650 billion so far to fight the Iraq war.

The Bush administration and the Federal Reserve announced an emergency rescue plan Sunday to bolster Fannie Mae and Freddie Mac, which hold or guarantee more than $5 trillion in mortgages — almost half of the nation's total.

The plan would temporarily increase a long-standing Treasury line of credit that could be provided to either company. Treasury also said it would, if necessary, buy stock in the companies to make sure that they have enough money to operate.

The Fed also announced that it would allow Fannie and Freddie to get loans directly from the Fed — a privilege previously granted only to commercial banks until March, when the Fed extended the borrowing to investment banks to deal with the collapse of Bear Stearns.

House Financial Services Chairman Barney Frank, D-Mass., predicted that Congress would grant approval for the extended line of credit as part of a broader housing measure that he predicted President Bush could sign by the end of next week.

Monday began with a good sign for Freddie Mac: It attracted more bidders than it had all year for one of its regular debt auctions, which raised $3 billion in short-term securities.

Fannie and Freddie stock rose early in the day but gave up the gains. Fannie closed down about 5 percent, at $9.73, and Freddie closed down about 8 percent, at $7.11.