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.