Most legal observers agree that U.S. District Judge Ginger Berrigan cut former Mayor Ray Nagin a big break Wednesday when she sentenced him to 10 years in a federal penitentiary, more than five years fewer than the low end of what federal sentencing guidelines dictated for his crimes.

It wasn’t that they were surprised Berrigan showed the former mayor some mercy. She’s generally viewed as the most liberal jurist on the federal bench in New Orleans, and she tends to hew less closely to the sentencing guidelines than do some of her colleagues.

But some were startled by just how drastically she departed from the guidelines, leaving open the possibility that federal prosecutors could successfully challenge the sentence should they choose to appeal it.

Once mandatory, the federal government’s sentencing guidelines are now advisory. But at the same time, judges don’t have unlimited discretion to stray from them, and appellate courts can overturn sentences if they find that a judge abused his or her discretion.

It’s unclear whether the government will appeal Berrigan’s decision. That is up to the Office of the Solicitor General, a branch of the Justice Department, which will rely in part on the guidance the office gets from local U.S. Attorney Kenneth Polite.

If the solicitor general greenlights an appeal, that challenge will be handled by Polite’s office. Polite had no comment on what his office might recommend.

Local attorneys say a number of factors could play into the government’s next move.

For starters, if Nagin files an appeal — as he and his lawyer, Robert Jenkins, have promised — it could have the effect of “lowering the bar” for a government cross-appeal, as prosecutors will already be mired in the appellate process. If Nagin were to leave well enough alone and accept his fate, the government might well do the same.

Tim Meche, a longtime federal defense lawyer who often is critical of prosecutors, said he thinks it is unlikely that the solicitor general will approve an appeal in the first place. Ever since the Supreme Court’s 2005 decision in the case Booker v. United States, which made guidelines advisory rather than mandatory, appeals of federal sentences are much rarer, Meche said.

Even if an appeal is filed, Meche said he wouldn’t bet on the 5th U.S. Circuit Court of Appeals overturning the sentence Berrigan imposed. At most, he said, the court might send the case back to her and ask her to bolster her case for giving Nagin a break.

“I think the chances of the government getting permission to appeal are slim to none,” Meche said. “Even if they appeal, I think the best they could hope for is that the Court of Appeals would send it back for her to give additional and better reasons for her sentence.”

On that point, Robert Weisberg, a law professor at Stanford University and an expert on sentencing guidelines, said he found some of Berrigan’s reasons for a more lenient sentence suspect. And though the reasons do not necessarily create legal precedent, they might well be cited or noted by other judges, creating precedent “in kind of a normative way,” he said.

Among Berrigan’s questionable reasons, in Weisberg’s view: her contention that, at 58, Nagin is older than most criminals whom sentencing guidelines are meant to deter from future crimes and her view that Nagin has been so thoroughly discredited that he’s unlikely ever to regain a position of public trust.

“You see older guys than him getting 20-year sentences for similar crimes, and in those cases, it becomes a life sentence,” Weisberg said. “But 58 is really not that old. And the argument that he will never again enjoy the public’s trust is an odd one. That’s true of almost anyone in this predicament. It’s kind of a double-counting. It’s pretty dubious — presumably anyone busted for a serious crime like that would be precluded from running for office.”

Of course, there is former Gov. Edwin Edwards, who is running for Congress at 86 after serving a 10-year corruption sentence that some viewed as a life term.

“There’s the entire human race, and then there’s Edwin Edwards,” Weisberg said. “And there’s (former Providence, Rhode Island, Mayor) Buddy Cianci,” who is also making a bid for his old job after a prison stint. “But they are outlier characters.”

Weisberg also thinks the sheer scale of the departure Berrigan granted — giving Nagin 36 percent less time than the low end of the guidelines — will factor into the solicitor general’s thinking.

“Departures are OK, but this was a large departure,” he said. “I’d say it’s 50-50 whether they appeal.”

Veteran defense lawyer Pat Fanning also thinks the scale of the departure could prompt an appeal.

“I don’t think she could have gone any lower,” he said. “She gave a one-third reduction to a guy who never accepted any responsibility. The solicitor general may think she went overboard.”

If an appeal is filed, Fanning thinks there’s a decent chance the conservative-leaning appellate court could overturn the sentence. “I think they got a real shot in the 5th Circuit,” he said.

Fanning thinks the critical question will be whether the government will file an appeal in the first place.

“You’ve got this kinder, gentler U.S. attorney and Department of Justice,” he said. “If you had Bush and (former Attorney General John) Ashcroft and (former U.S. Attorney) Jim Letten, I think they’d be almost certain to appeal. But I don’t know with this group. They’re the ones pushing for people to do less time in some cases.”

Follow Gordon Russell on Twitter, @gordonrussell1.