Shares of General Motors Corp tumbled as much as 39 percent to hit a 70-year low on Thursday as prospects dimmed that lawmakers would reach a compromise on a proposed $25 billion bailout for U.S. automakers before Congress adjourns this week.

Shares of Ford Motor Co (F.N) hit their lowest level in more than 26 years, and auto parts supplier stocks declined across the board amid concerns a failure by one of the U.S. automakers would touch off a cascade of failures in the struggling industry.

"We're still analyzing the situation, but the feeling is that without a loan package, the probability of GM or Chrysler going bankrupt in early 2009 is extremely high, at about 75 percent," said George Magliano, a forecasting director at influential auto industry tracking firm Global Insight.

Even with a government bailout, Magliano said he saw a 25 percent change of bankruptcy for either automaker amid the credit crisis that has pushed U.S. auto sales to 25-year lows.

Shares of GM were down 19 percent, or 53 cents, to $2.26 on the New York Stock Exchange, after falling earlier in the session to $1.70, their lowest level since 1938.

Shares of Ford fell 9 percent, or 10 cents to $1.16. The stock fell as low as $1.02.