Ten days into his tenure as United States attorney in Manhattan, Preet Bharara saw his political and prosecutorial worlds collide. He convened a meeting to discuss a sensitive investigation of a Democratic donor with ties to Senator Chuck Schumer of New York. Mr. Bharara had been Mr. Schumer's chief counsel, and Mr. Schumer had recommended Mr. Bharara for the prosecutorial post. At the meeting, Mr. Bharara asked his prosecutors if there was enough evidence to make a case against the donor, Hassan Nemazee. One of the prosecutors, Daniel W. Levy, who is now in private practice, would recall years later...