The 55-month sentence sparked an outcry, with critics saying it was far too lenient. Prosecutors appealed, saying Buckwalter's decision was shot through with procedural mistakes.
The appellate judges appeared to agree.
"It sounds like a very fundamental error," Judge Julio M. Fuentes said after Assistant U.S. Attorney Robert A. Zauzmer cited one of many procedural mistakes that the prosecutor said made it legally necessary for Buckwalter to be ordered to resentence Fumo.
Judge Leonard I. Garth also said the sentencing decision by Buckwalter did not contain the required explanation of the steps taken to reach the 55-month sentence.
"He was supposed to explain it," Garth said of Buckwalter's decision, "but I don't find it in the record."
The third member of the appeals panel, Richard Nygaard, was silent throughout the session.
Defense attorney Samuel Buffone did not directly address the issue Fuentes and Garth raised, but said the massive record of the trial and exhaustive sentencing hearing provided plenty of justifications for the judge's thinking.
"They were all there," he said. "What Judge Buckwalter did here may have been a bit unusual, but it was well within his discretion."
Once one of the most influential Democratic politicians in Philadelphia and Harrisburg, Fumo, 68, has now served 21 months behind bars in Kentucky. With time off for good behavior, he could be released in January 2013.
Fumo was not allowed to attend the hearing Wednesday, but his fiancée and two of his three adult children were there. A third child is estranged.
After a five-month trial, Zauzmer and fellow federal Prosecutor John J. Pease persuaded the jury that Fumo had turned his Senate staff into servants and political minions who did errands for him on state time. The jury also found that he had defrauded a pair of nonprofit organizations.
After Fumo realized that an FBI team led by Agents Vicki Humphreys and Kathleen McAfee was on his trail, the jury found, he tried to obstruct the probe.
The wrongdoing cost taxpayers and the nonprofit groups more than $2 million.
Since the U.S. Supreme Court struck down mandatory-sentencing laws in 2005, judges have been given much wider power to decide punishments.
But under court rules, judges must still compute the sentences that convicted defendants would have faced under the guidelines and explain any decision to depart from them.
Procedural mistakes by judges can open the door for prosecutors to attack their entire decisions and demand a do-over.
"We have just error after error, a confluence of errors that are piled up on top of each other," Zauzmer said.
The sentence finally imposed was "outrageous," he said, adding that rectifying the mistakes at a new sentencing hearing would "point to a much longer appropriate sentence."
Zauzmer ripped into Buckwalter's decision to cut Fumo a break in sentencing on the grounds, as the judge put it, that Fumo was an "extraordinary" public servant.
In fact, Zauzmer said, the FBI investigation had shown that what was extraordinary about Fumo was his months and months away from work. In fact, Zauzmer said, Fumo was no more than a "part-time legislator" who engaged in "stealing every single day from the public."
Judges Fuentes and Garth never commented directly on the justice of Fumo's sentence, but they repeatedly suggested that Buckwalter had made serious mistakes.
"I don't understand how the 55-month sentence was arrived at," said Garth, a senior judge.
The two judges complained that Buckwalter had failed to adequately explain his reasoning on key issues - such as his dismissal of the prosecution's contention that Fumo deserved extra time for masterminding a particularly sophisticated criminal scheme and for ripping off charitable organizations.
They also appeared to fault Buckwalter because he simply gave up calculating for sentencing purposes what prosecutors said was Fumo's awarding of a $150,000 no-work contract to a crony.
That omission, Garth said, appeared to be a "clear error."
If Buckwalter presides over a new sentencing hearing, he conceivably could fix any computation errors, offer a more detailed rationale for his decision, and impose the same 55-month sentence.
But prosecutors would get a chance to argue anew for tougher sentencing and to assemble fresh evidence about the gravity and cost of Fumo's wrongdoings.
Though most of Wednesday's debate turned on the issue of the sentencing, the judges also heard some argument concerning Fumo's separate appeal. He has asked for a new trial, asserting in part that Buckwalter should have held a special hearing to determine the truth of a report that at least one juror had learned of Fumo's 1980 corruption indictment, which was dismissed on appeal.
But Zauzmer argued that the judge was right to shut the door on exploring that matter. He said its potential impact did not warrant the major step of calling jurors back after a verdict to explore their conduct.
Garth was selected in 1973 by President Richard Nixon; Nygaard, also a senior judge, was nominated in 1988 by President Ronald Reagan; and Fuentes was named to the bench by President Bill Clinton in 1999. Nygaard is from Western Pennsylvania; the other two are from New Jersey.
Ever since a 2005 U.S. Supreme Court decision striking down mandatory-sentencing guidelines, federal judges have had broad power to punish as they see fit.
But they must still follow strict procedural rules, including provisions that they calculate what the guidelines recommend, and then explain any departure from those guidelines.
After a jury convicted former State Sen. Vincent J. Fumo on all 137 corruption counts in 2009, prosecutors and the U.S. Probation Office agreed that the guidelines called for him to get 21 to 27 years in prison.
But the U.S. district judge in the case, Ronald L. Buckwalter, came up with a far different and lower calculation. He cut the figure in half, saying Fumo would face a range of 11 to 14 years behind bars.
To get to that lower figure, prosecutors say in appeals, Buckwalter improperly lowballed the fiscal cost of Fumo's crimes. They said his theft cost taxpayers and nonprofits more than $4 million, but that the judge had come up with a figure roughly half that.
Under the guidelines, the more criminals steal, the more recommended time they face.
Prosecutors said Fumo should face even more time under guidelines that punish especially egregious conduct. They said he deserved extra punishment, for instance, for carrying out an especially sophisticated crime - a sanction called for in the guidelines.
But Buckwalter said Fumo's crimes couldn't have been all that clever given that some of his misdeeds had been spotlighted by journalists.
Prosecutors replied that the FBI had to dig long and hard to unravel the full scope of his wrongdoing, turning up violations that had never before been publicly revealed.
Once Buckwalter had come up with his sentencing range, he still wasn't done.
In the end, he imposed a sentence of just four years and seven months. The judge said Fumo was due the reduced 55-month sentence in part because he had been an effective legislator.
"You worked hard for the public, and you worked extraordinarily hard," the judge said in court, "and I'm therefore going to grant a departure from the guidelines." - Craig R. McCoy
Contact staff writer Nathan Gorenstein at 215-854-2797