In addition, speculators who think a financial firm will fail can buy credit-default swaps. They will profit if they are right. But even if the firm is not in trouble, an increase in the price of those swaps may scare other investors, and send the company’s stock down. That prospect has alarmed the S.E.C. As the political debate was growing, the International Swaps and Derivatives Association, a trade group, reported that the amount of outstanding credit-default swaps declined in the first half of 2008, something that had never happened before.

The 12 percent decline, to $54.6 trillion, still left the market vastly larger than the total amount of debt that can be insured. The huge total reflects the way the market is structured, as well as the fact that someone does not need to actually be owed money by a company to be able to buy a credit-default swap. In that case, the buyer is betting that the company will go broke.

Within that huge market, many contracts offset one another  assuming that all parties honor their commitments. But if one major firm goes broke, the effect could snowball as others are unable to meet their commitments.

In regulated futures markets, contracts are centrally cleared. If you buy an oil futures contract on Monday, and sell it on Wednesday, you have made your profit (or taken your loss) and you no longer have any stake in whether oil prices rise or fall. But if you buy a credit-default swap on Monday from one firm, and sell an identical swap on Wednesday to another firm, you still face the potential of risk if the party that sold the swap to you is unable to pay when a default occurs, perhaps years later.

“One of the major reasons that the government helped out in the Bear Stearns situation,” Treasury Secretary Henry M. Paulson Jr. testified at a Senate hearing this week, “was to avoid throwing it into bankruptcy with all the credit-default swaps.”