The risks associated with the vast, unregulated market for credit default swaps played a crucial role in the bailout of Bear Stearns. Now these financial instruments are taking center stage in another Wall Street drama: whether regulators will let MBIA, the big bond insurance company, renege on a promise to shore up a crucial unit with $900 million in capital.

MBIA has written $137 billion in swaps, which are privately traded insurance contracts that let people bet on companies’ financial health. Most of these contracts stipulate that if MBIA’s bond insurance unit becomes insolvent or is taken over by state regulators, buyers can demand payment immediately.

But if that were to happen, MBIA would have far less money to pay policyholders and owners of municipal bonds backed by the company. So the swaps give MBIA significant leverage over Eric R. Dinallo, the commissioner of the New York State insurance department, who wanted the company to bolster its insurance unit with the $900 million in cash.

In the case of Bear Stearns, the Federal Reserve feared that credit default swaps might unleash a chain reaction of losses if the bank were allowed to collapse. Given the threat that similar swaps may pose to MBIA, Mr. Dinallo is unlikely to push for a regulatory takeover of the subsidiary even if Joseph W. Brown, MBIA’s chief executive, refuses to recapitalize the unit.