But now Cain is under criminal investigation and has begun cooperating with the U.S. Justice Department in its 139-count corruption indictment against Fumo, according to sources quoted in an Inquirer story yesterday.

Cain, 59, began cooperating with the investigators after he came under scrutiny for possible tax violations, the sources said. If he testifies against Fumo, Cain would be the first of the inner circle to give evidence against their mentor.

In recent months, Cain has told associates that he and Fumo had quarreled. The issue was whether Fumo, wounded by the corruption probe, could nevertheless win reelection this fall. Cain thought not, a view he believed irked the senator.