As federal prosecutors opened their bribery and corruption case Tuesday against former Philadelphia Sheriff John D. Green, defense lawyers said the case may come down to a battle of credibility – between the sheriff on trial and the one who followed him into the office.

Former Sheriff Barbara Deeley is expected to testify under a grant of government immunity to support allegations that Green — throughout his two-decade tenure in office — accepted bribes worth six figures from a friend and campaign donor who was granted a virtual stranglehold over lucrative Sheriff's Office contracts.

But Lewis S. Small, a lawyer representing that donor – James R. Davis Jr. — wasted no opportunity Tuesday to discount Deeley's testimony before she could take the witness stand.

"Barbara Deeley is a perjurer, a liar, a felon," the attorney said, in his opening remarks to the panel of seven men and five women hearing the case. "You'll hear her testimony. You won't believe a thing she has to say."

Green's lawyer, Peter Scuderi, also urged jurors to withhold judgment, promising they would hear from his client directly later in the trial.

The two lawyers' remarks – on the first day of what is expected to be a three-week trial – promised a messy and highly public courtroom showdown between the former city row officers and onetime allies that has been simmering behind closed doors for years.

Deeley had a front-row seat to much of the chicanery, said Assistant U.S. Attorney Jennifer A. Clarke. She worked for years as Green's chief of staff, ran his 2007 campaign, and succeeded him on an interim basis after he abruptly retired in 2010.

Her firsthand view of the relationship between Green and Davis — who also is charged in the case — makes her a valuable witness for the government.

But Green — a 17-year Philadelphia Police Department veteran who bucked the city's Democratic Party to run his own upstart campaign for sheriff in 1987, only to hold the post for the next 22 years — maintains there was nothing sordid about his ties to Davis.

In fact, Scuderi said Tuesday, Green's office had awarded the contracts at the heart of the case — for management of court-ordered sales of foreclosed properties — to Davis' firms long before any of the gifts referenced in the indictment.

"If John Green is a criminal," Scuderi said, "he's a lousy criminal."

In her own opening statement to the jury, Clarke described the bond between Davis, 67, and Green, 70, as based entirely on what each could give the other.

In Green's case, that meant gifts of hidden campaign donations; income for his wife, and financial support from Davis, who was happy to keep giving as long as his companies continued to hold contracts that paid him millions more, Clarke said.

When Green considered retiring before his 2007 race for reelection, she said, it was Davis who persuaded him to run again by offering to cover the majority of the campaign's costs.

"It was essential to Davis that he keep the business [with the Sheriff's Office] because it made up more than 90 percent of his income," Clarke said. "And it was essential that John Green stay in office to give it to him."

Davis, as prosecutors have described it, asserted himself as a force in Green's campaigns, allegedly providing more than $200,000 in contributions, labor, and advertising costs – little of which was reported in campaign finance filings.

He gave Green's wife a job and even helped the sheriff pay for two houses – an East Mount Airy property that he sold to Green in 2002 for a $12,000 loss, and later a retirement home near Kissimmee, Fla., for which Davis helped pay with gifts and loans totaling $320,000.

Green, meanwhile, continued to contract with Davis' companies – Reach Communications Specialists Inc., an advertising firm,; and RCS Searchers Inc., a title company – to manage one of the Sheriff's Office's chief responsibilities.

City audits have shown that those businesses were paid hundreds of thousands of dollars per month for placing ads in local newspapers to advertise sheriff's sales and managing title transactions, with Davis earning a 15 percent commission.

And when Reach added on excess fees totaling $7 million that were not part of the initial deals, Green unquestioningly paid the bills, prosecutors said, depleting the amount of money from each sale that went back to the original homeowner who had lost the house.

Small, Davis' lawyer, defended those fees Tuesday.

"There were no bribes," he said. "There was no quid pro quo."

It was Deeley who ultimately canceled Davis' contracts when she was appointed interim sheriff after Green's retirement.

But prosecutors suggested Tuesday that that was not always part of the plan.

In her opening remarks, Clarke accused Davis of flying both Deeley and her eventual replacement, current Sheriff Jewell Williams, to Florida for a 2010 meeting.

There, they allegedly coordinated with Green to stage-manage his departure from office while ensuring a line of succession that would protect Davis' business with the Sheriff's Office.

Prosecutors have not accused Deeley or Williams of any crimes. Both Green and Davis have pleaded not guilty to counts including conspiracy and honest services fraud.

Their trial is expected to resume Wednesday.