A longtime confidant of state Sen. Vincent J. Fumo was charged this morning with tax evasion in a move by federal prosecutors that set the stage for him to be a government witness against the powerful South Philadelphia Democrat.

Howard J. Cain, 60, is expected to plead guilty sometime in the next few weeks and to take the stand this fall when Fumo stands trial in a sweeping federal corruption case. Cain was charged with one count of tax evasion.

Cain, a top Philadelphia political strategist for many years, is the only person from Fumo's historically tight inner circle to defect and become a prosecution witness.

In court documents, federal prosecutors said that Cain failed tax returns with the IRS going back to 1991 on income totalling $1.6 million. The charges say that owes $411,303 in back taxes.

The plea agreement signed by Cain, his attorney, Peter J. Scuderi and by Assistant U.S. Attorneys John J. Pease and Linda Dale Hoffa, chief of the criminal division, states that Cain will plead guilty and "cooperate fully and truthfully" with the government.