A longtime confidant of State Sen. Vincent J. Fumo's pleaded guilty yesterday to tax evasion and, in a move that could get him a break at sentencing, agreed to testify as a prosecution witness during Fumo's corruption trial.

Howard J. Cain, 59, a political strategist and veteran of more than 80 campaigns, admitted that he had neither filed a federal tax return since 1991 nor paid any federal taxes on more than $1 million of income from 1997 through 2006.

His defense lawyer, Peter J. Scuderi, said that it had been a "difficult decision" for Cain to cooperate against Fumo, but that he had "sworn to tell the truth" as a cooperating witness.

"He is guilty, and he realizes he was wrong, and he intends to testify truthfully in court," Scuderi told reporters outside the U.S. District Courthouse at Sixth and Market after the half-hour proceeding.

Fumo's defense lawyer, Dennis J. Cogan, declined to comment about the significance of Cain's cooperation - or Fumo's reaction to it. "I'll have plenty to say at trial about it," he said late yesterday.

Fumo, the Philadelphia Democrat who has been one of the most powerful lawmakers in Harrisburg for decades, is accused of defrauding the state Senate and two nonprofit organizations, and of attempting to obstruct the FBI and IRS investigation.

Trial of the corruption case, scheduled to start Sept. 8, is expected to last at least three months, with more than 100 prosecution witnesses.

Cain, a longtime member of Fumo's famously tight inner circle, has now emerged as a potentially key witness.

According to Cain's guilty-plea agreement, he admitted that he engaged in a conspiracy with Fumo and others to defraud the state Senate by submitting false invoices that resulted in payments to Cain for engaging in political campaign work in violation of state law.

Assistant U.S. Attorney John J. Pease said Cain faces 37 to 46 months in prison under federal sentencing guidelines before any credit for his cooperation.

If they are satisfied that Cain has fulfilled all of his obligations as a cooperating witness, the plea agreement states, prosecutors could ask U.S. District Judge Michael M. Baylson to hand down a shorter sentence. Baylson scheduled sentencing for Jan. 26.

"Why are you pleading guilty?" Baylson asked Cain.

"I did not file my income taxes," replied Cain.

According to the charges, Cain had income of more than $1.6 million from 1997 through 2006 and owes more than $411,000 in back taxes, not including penalties.

Pease said that Cain's clients paid him through his consulting business, and that Cain was able to evade taxes by failing to have the business submit 1099s or W-2 forms to the IRS.

Pease said that the Senate Appropriations Committee - which Fumo long chaired - was Cain's top client. In all, court documents stated, the state Senate paid Cain more than $500,000 from 2000 through 2006.

When the FBI and IRS raided Cain's home earlier this year, Pease and Assistant U.S. Attorney Robert A. Zauzmer stated in court documents, they found plenty of evidence about his income, along with information that he had been in contact with an accountant.

The accountant, in turn, told investigators that Cain approached him in 2004 after getting subpoenaed to testify before the federal grand jury in the Fumo investigation because he was "concerned" the government might find out that he hadn't filed any tax returns since 1991. The prosecutors said that the accountant prepared draft tax returns, but that Cain never followed through.