The legacy of Preet Bharara’s crackdown on insider trading — his prosecutors won more than 80 convictions as they swept through Wall Street — might now hinge on the most unpredictable of allies: Congress.

On Friday, the United States Court of Appeals for the Second Circuit denied Mr. Bharara’s request to reconsider a ruling in December that sharply narrowed the definition of insider trading. That ruling, issued by a three-judge panel of the court, tossed out the convictions of two hedge fund traders and threatened to dissolve other signature convictions and pleas secured by the office of Mr. Bharara, the United States attorney for Manhattan.

The latest denial from the appeals court now leaves Mr. Bharara with few legal options. With the approval of the United States solicitor general, Mr. Bharara could seek to appeal the ruling to the Supreme Court, but legal analysts have described his chances there as a long shot.

Given the paucity of legal avenues in this landmark case, the answer for future investigations might lie instead with Congress, which could clarify just what constitutes insider trading. Already, in the wake of the December ruling, lawmakers have introduced at least three bills that would establish clearer ground rules, potentially empowering prosecutors and stemming the fallout from the ruling.