PHILADELPHIA — Attorneys for long-powerful state Sen. Vincent Fumo are battling efforts by federal prosecutors to increase his 4 1/2-year prison sentence in a corruption case even as they seek a new trial, saying he was denied an impartial jury.

Defense attorneys argue that U.S. District Judge Ronald Buckwalter was on solid legal ground in imposing the sentence, which federal prosecutors called “unduly lenient” in an appeal.

“If the convictions are not reversed, the sentence must be affirmed,” the defense argued in documents filed Friday.

Prosecutors had sought at least a 15-year term for Fumo, a 67-year-old Philadelphia Democrat who was convicted last year of 137 fraud and obstruction counts after a five-month trial.

Authorities said Fumo defrauded the state Senate, a museum and a nonprofit of millions of dollars.

In their appeal, they said “breathtaking” corruption had been exposed in the case and said Fumo over the course of three decades in the state Senate used his control of a Senate panel and one nonprofit and influence over another “to support a lavish lifestyle and illegally amass political power.”

Prosecutors argued in their appeal that Fumo should have faced up to 21 to 27 years in prison and that the amount of fraud was greater than $4 million, twice the total assessed by the judge. They also said the judge failed to explain how he calculated the fraud total or sentencing range.

Fumo’s attorneys, however, said the district court committed “no significant procedural error” in sentencing him to 55 months’ imprisonment followed by three years of supervision, a $411,000 fine and more than $2.3 million in restitution.

Saying the government was arguing “narrow issues of procedural error,” they said the amounts had been calculated fairly and the decision to credit Fumo for “good works” was well grounded.