Advocate staff file photo by JOHN McCUSKER -- Rodney Williams was a star witness against Ray Nagin's in the former mayor's trial in January.

Businessman Rodney Williams, the star witness in the government’s case against former Mayor Ray Nagin, is set to be sentenced in federal court Wednesday.

Under the terms of a 2012 plea agreement, Williams faces a maximum of 37 months in prison, but federal prosecutors are expected to ask U.S. District Judge Susie Morgan for a much more lenient sentence, or perhaps even probation. Morgan met with prosecutors and Williams’ lawyers, Ralph and Brian Capitelli, Monday to discuss Williams’ sentence.

Williams, who founded the engineering firm Three Fold Consultants, pleaded guilty in December 2012 to bribing Nagin and his sons in exchange for the mayor’s help in securing no-bid city contracts as the city embarked on a huge rebuilding program in the wake of Hurricane Katrina.

The plea deal was the last major piece of the puzzle in the conspiracy case the feds built against Nagin. About six weeks after Williams admitted his guilt, Nagin was charged in a 21-count indictment with trading his influence in exchange for cash and gifts from Williams and others.

Williams was especially important to the case because he was the only key witness who was untainted save for his interactions with the mayor, and was thus perhaps the most credible of Nagin’s co-conspirators to testify.

He portrayed himself as a businessman unable to get much work from the city until he agreed, along with his two partners in Three Fold, to pump $60,000 into the Nagin family’s granite business, Stone Age. The partners did so with the understanding that the doors of City Hall would open to them if they did, and soon, Three Fold was getting dozens of contracts, he testified.

The Nagins later created documents that Williams said were bogus that indicated the Three Fold partners shared a 4.5 percent interest in Stone Age.

Though each partner put up $20,000 for that phony stake, Williams, who dealt with Nagin directly, was the only one of the three charged. Williams later said he also ponied up another $12,250 in bribes to Nagin’s sons.

The other major witnesses in the case against Nagin — former city Chief Technology Officer Greg Meffert, mortgage broker Michael McGrath and businessman Frank Fradella — each were compromised in that all three faced the prospect of long prison sentences for schemes that did not involve the mayor.

McGrath, in fact, testified in manacles and an orange jumpsuit, the get-up he dons daily as he serves a 14-year sentence for an audacious, $140 million mortgage fraud.

When Nagin took the stand, he disparagingly referred to McGrath as “that guy in the orange jumpsuit.” That reference, while sarcastic, highlighted the linchpin of the mayor’s defense strategy: to cast aspersions on the government’s witnesses, suggesting they were simply seeking to reduce their own exposure by helping the case against Nagin, not telling the truth.

There’s no dispute the witnesses were motivated. Thanks to his cooperation, Meffert was sentenced last week to 30 months in prison, far less than the eight years that federal sentencing guidelines called for in his case. A day later, Mark St. Pierre — who also cooperated after his 2011 conviction but did not testify against Nagin — saw his 171/2-year sentence trimmed to five years as thanks for his help.

Meffert and St. Pierre were convicted in a scheme in which Meffert routed city contracts worth millions of dollars to St. Pierre, receiving more than $860,000 in kickbacks in return.

McGrath, who is three years into a 14-year sentence, has not received a break on his sentence, but he may yet get one.

Fradella, who pleaded guilty to orchestrating a huge stock fraud as well as to bribing Nagin, is not due to be sentenced until December. He faces a maximum of five years in prison.

Nagin, meanwhile, reported to a prison camp in Texarkana, Texas, on Monday to begin serving a 10-year sentence. He was convicted by a jury in February on 20 of the 21 corruption-related counts he faced.

Editor’s note: This story was changed Sept. 10 to reflect that Frank Fradella’s sentencing is set for December rather than January.

Follow Gordon Russell on Twitter, @gordonrussell1.