In early 2005, General Motors' chief financial officer dismissed some of the darker speculation about his company, saying, "The idea of bankruptcy is nuts."

Such talk has sounded more and more sane as the company struggles with huge losses and sagging sales. Last year, G.M. lost $38.7 billion.

"Bankruptcy is not impossible," wrote John Murphy, an analyst with Merrill Lynch in cutting his recommendation on G.M.'s stock to "underperform" from "buy."

Shares of G.M., already at lows not seen since the 1950s, tumbled 13 percent today. The stock is down more than 70 percent over the last 12 months.

"The key change in our outlook is a much lower forecast for U.S. auto sales that is driving a higher cash burn, necessitating a much larger capital raise than the market is currently anticipating," Murphy wrote. "Furthermore, we believe there is potential downside in the stock below $7 and that bankruptcy is not impossible if the market continues to deteriorate and significant incremental capital is not raised."

The analyst's outlook came a day after the auto industry reported very weak sales for June. G.M.'s sales were better than expected, but that came largely on the back of zero-percent financing incentives.

Talk of a bankruptcy filing haunted G.M. in the early 1990s, and it plagues Chrysler under its new private-equity owners. (Chrysler said last week that it had no plans to file for bankruptcy protection.)

For G.M., there is no danger of a bankruptcy anytime soon: The company had $24 billion in cash or cash equivalents at the end of its first quarter.

But Murphy believes that G.M. will need to raise as much as $15 billion to shore up liquidity.

If it ever happens, a bankruptcy filing would be a cataclysmic event. Thousands of suppliers and other companies depend on G.M. It is also the largest issuer in the corporate debt market. A bankruptcy would set off a chain reaction that would ripple through the economy and the credit markets.