In a massive filing by his lawyers, imprisoned former State Sen. Vincent J. Fumo fought back Friday against a bid by federal prosecutors to lengthen his 55-month prison term.

Locked up for 16 months now, Fumo, 67, argued through his lawyers that U.S. District Judge Ronald L. Buckwalter was on firm legal ground in handing out the sentence, widely criticized as too lenient in a furor that broke out afterward.

Buckwalter, the defense team wrote in its 172-page appeals brief filed late Friday night, "committed no significant procedural error" in the sentencing and had the authority to fashion a sentence as he saw fit.

Yet while praising Buckwalter's handling of Fumo's punishment, Fumo's attorneys said the judge had mishandled the trial.

They asked the U.S. Court of Appeals for the Third Circuit to grant a new trial. The defense contended, among other complaints, that one juror had improperly picked up negative information about Fumo, learning from coworkers that Fumo had been convicted of corruption in a 1980 verdict that was set aside after appeal.

The warring sides get to exchange one more round of briefs over the next two months and then will likely make their cases before a panel of Third Circuit judges, perhaps in the spring. Fumo may know his fate in the dispute by late next year.

Painting a glowing portrait of the Democrat, his lawyers described him as a "champion" of the people who had responded to a "call to service" after the assassination of John F. Kennedy and had been a tireless legislator for 30 years.

They noted that Buckwalter had termed Fumo's public service "extraordinary" and said the judge had been entirely justified in cutting the politician a break in sentencing to reflect that record.

After a marathon trial, prosecutors got a jury to adopt a different view of Fumo - as a greedy lawbreaker who spied on his political enemies and even his children.

The panel convicted Fumo on all 137 counts in an indictment that charged him with defrauding a pair of nonprofit organizations, turning Senate employees into servants and taxpayer-paid political operatives, and then staging a cover-up in a failed attempt to thwart an FBI investigation.

The Fumo attorneys are Samuel J. Buffone, Dennis J. Cogan, and Peter Goldberger.

Assistant U.S. Attorneys Robert A. Zauzmer and John J. Pease had sought a sentence of more than 20 years and want the appeals court to send the case back to Buckwalter, who they hope would give longer terms to Fumo and his codefendant, Ruth Arnao. A longtime aide, she was convicted of all 45 counts and sentenced to one year in prison. Arnao, 54, has already served that time.

Zauzmer and Pease, who declined comment Saturday, may have an uphill fight. The U.S. Supreme Court struck down mandatory sentencing laws in 2005, and since then appellate courts have been reluctant to question the discretion given trial judges to determine punishments.

So the U.S. Attorney's Office must make its case on procedural grounds, showing that Buckwalter miscalculated or misconstrued federal sentencing guidelines or made other serious legal errors.

The government says Buckwalter erred by not penalizing Fumo for exploiting charities and for not deeming him the perpetrator of a "sophisticated" crime. Federal sentencing rules permit this to be used to justify tougher sentences.

On the charity issue, the defense argued that it was far from clear that Fumo had a criminal intent when he set out to raise money for Citizens' Alliance for Better Neighborhoods, one of the two nonprofits he was convicted of ripping off.

Fumo pressured Peco Energy to secretly freeze its rates and make a charitable contribution of $17 million to Citizens' Alliance as part of a deal under which he dropped his opposition to Peco's business plans.

The defense also argued that the crime was not especially complex. Further, the defense said, weighing the sophistication of Fumo's misdeeds was up to Buckwalter, who heard all the evidence, not the appellate court.

In a 219-page filing, the two federal prosecutors had said Buckwalter's decision to reduce the cost to taxpayers of Fumo's rip-offs from $4 million to $2.4 million, "had the effect of slicing Fumo's [sentence] guidelines range in half."

Reducing the value of Fumo's crimes was especially significant because the recommended prison term increases dramatically once a theft exceeds $2.5 million. Thefts over that threshold expose a defendant to four extra years.

The defense defended Buckwalter's loss calculation. And even if some of the prosecutors' arguments were correct, the defense said, the loss was still below $2.5 million.

But only by a fraction. At one point, the defense, giving some ground to the prosecution for the sake of argument, said the loss could be pegged at $2,499,788.22.

Jurors found that Fumo had turned his Senate staff into personal and political servants and had defrauded Citizens' Alliance, a civic-improvement group led by Arnao, and the Independence Seaport Museum, of which Fumo was a board member.

Before sentencing Fumo, Buckwalter criticized the FBI, prosecutors, and the news media.

"It's not murder. It's not robbery. It's not even assault," Buckwalter said at Fumo's sentencing. "It's nothing violent. It's not the selling of a political office."

He noted receiving 259 letters recommending leniency for Fumo and only five critical of the former senator. Prosecutors said most of the pro-Fumo letter writers were relatives, friends, and political and business associates.

Those guidelines require judges to enumerate a number of factors, including the nature of the crime, the amount of money taken, and the typical range of sentences for similar wrongdoing before imposing punishment.

Prosecutors had agreed with the federal Probation Office conclusion that Fumo should go to prison for 21 to 27 years.

But Buckwalter interpreted the guidelines differently, finding the sentence range to be just 10 to 14 years. Beyond that, he gave Fumo a huge break for his public service.

It prompted a public outcry.

In July, prosecutors said in their brief that "common thieves" routinely receive harsher sentences than Fumo, who engaged in "breathtaking" corruption.

"It is likely impossible to identify a defendant in recent years who stole over $2 million, abused a position of public trust, and obstructed justice in the process, who received a sentence anything like Fumo's," they wrote.

Arnao is not fighting her conviction, and, unlike Fumo, has said she was sorry for doing wrong. However, her lawyers, too, filed a brief defending Buckwalter's sentencing.

Patrick Egan, one of her lawyers, said Buckwalter's decisions had been "well within" a judge's discretion.

In his brief, Egan wrote that Arnao's responsibility for the crimes was far less than Fumo's because she had developed a "dependent personality disorder" that allowed Fumo to exert unusual influence over her.

"Arnao indeed did organize her life in a submissive fashion. . . . His welfare became her reason for being," her attorneys wrote, and quoted a statement by her that said: "I felt too weak to either try to overrule Vincent or just walk away."

Contact staff writer Craig R. McCoy at 215-854-4821 or cmccoy@phillynews.com.