In a massive legal 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 now for 16 months, Fumo 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 after sentencing.

Buckwalter, the defense team wrote in their 172-page appeals brief, "committed no significant procedural error" in the sentencing.

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

They asked the Third Circuit Court of Appeals to grant Fumo a new trial. The defense contended, among other complaints, that jurors had improperly learned negative information about Fumo - that he had been previously convicted of corruption in a previous verdict set aside after appeal.

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

They noted that Buckwalter had termed Fumo's public service "extraordinary" and said that the judge was 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. They panel convicted him on all 137 counts leveled after Fumo in a sweeping indictment that charged him with defrauding a pair of nonprofit organizations, turning state Senate employees into servants and then staging a cover-up in a failed attempt to thwart an FBI investigation.

Any decision by the Third Circuit is likely months away, as the appellate court will have to sort through hundreds of pages of highly detailed briefs and appendices submitted by the two sides.

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 originally sought a sentence of more than 20 years and now 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. She was a longtime aide who was convicted of 45 counts and was sentenced to one year in prison. Arnao has already served that time.

However, Zauzmer and Pease may have an uphill fight. Mandatory sentencing laws were struck down by the U.S. Supreme Court 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 exploting 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 raised millions for Citizens' Alliance for Better Neighborhoods, one of the two nonprofit he was convicted of ripping off. Fumo struck a secret deal with Peco Energy under which it gave $17 million to Citizens' Alliance and froze its rates while Fumo agreed to drop his opposition to its business plans.

The defense also argued that the fundamental nature of Fumo's crime was not especially complex. The defense also said that it was up to Buckwalter, who heard all the evidence, to weigh the sophistication or lack of it of Fumo's misdeeds, not the appellate court.

In a massive filing of their own, the two federal prosecutors had previously 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 monetary 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.

But the defense argued that, even if Buckwalter made mistakes in his calculation, the loss still fell below the $2.5 million threshold.

But only by a fraction. At one point, the defense put the loss at $2,499,788.22.

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

During the trial, Buckwalter criticized the FBI, prosecutors, and the news media.

"It's not murder. It's not robbery. It's not even assault," Buckwalter said of Fumo's wrongdoing at the time of his 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 state senator. Prosecutors said that most of the pro-Fumo letter writers were family, 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.

In Fumo's case, prosecutors had agreed with the federal Probation Office conclusion that he 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 268-page 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.

Lawyer for Arnao did not fight her conviction but did file a brief defending Buckwalter's sentencing.

Patrick Egan, one of her lawyers, argued that Buckwalter's decisions were "well within" the discretion afforded him as a judge.

In his brief, Egan wrote that Arnao's responsibility for the crimes was far less than Fumo's because she 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 Nathan Gorenstein at 215-854-2797 or ngorenstein@phillynews.com.