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Mr. McDonnell, meanwhile, has said the gifts were motivated by friendship and that the family has repaid them. His team also argues that nothing was ever promised or given to Mr. Williams, and they have secured an accountant as an expert witness to rebut charges that the McDonnells were struggling financially.

Whether Mr. McDonnell decides to take the stand and make that case himself will be one of the key questions, said Randall Eliason, a former assistant U.S. attorney for the District of Columbia who specialized in white-collar crime.

“I think the jury naturally wants to hear from the defendant in a case like this,” he said. “As a prosecutor, most of the time you’re praying the defendant does take the stand in a case like this, because most of the time it doesn’t go well. They’re not used to cross-examination, and there’s really nothing like that. It’s definitely a high-risk decision.”

The courts denied a motion to try Mr. and Mrs. McDonnell separately. Mrs. McDonnell had argued she would offer testimony in a separate trial but that she would not if tried jointly with her husband.

Regardless of whether he does testify, Mr. McDonnell and his team will likely rely heavily on his longtime reputation as a squeaky-clean politician and former prosecutor himself who had frequently been mentioned as a possible vice presidential pick for 2012 GOP presidential nominee Mitt Romney.

“Throughout his political career, I think people thought Bob McDonnell was a straight arrow — not a politician who would push the edges on corruption at all,” Mr. Holsworth said.

Mr. McDonnell’s brand apparently had such cachet that even as more details about his relationship with Mr. Williams were unfolding last year, Gov. Terry McAuliffe’s campaign actively discouraged state Democrats from openly criticizing the Republican as Mr. McAuliffe tried to advance a campaign theme of bipartisan cooperation.

Court filings indicate that part of the argument could also frame Mr. McDonnell as unaware of some of the dealings between his wife and Mr. Williams, notably that of the Rolex watch. The defense was granted 20 blank subpoenas in June for potential witnesses, which range from members of Mr. McDonnell’s family to an unnamed out-of-state public official.

The huge wild card, said state Sen. J. Chapman “Chap” Petersen, is Mr. Williams, who has been granted immunity by federal prosecutors and could drop additional bombs on the McDonnells’ political and personal lives over the course of the trial.

“For the first time, we’re going to hear from him publicly,” said Mr. Petersen, Fairfax Democrat.

Mr. Petersen was the first, and one of the only, elected officials in either party to call on Mr. McDonnell to fully disclose his dealings with Mr. Williams or step down from office.

“In retrospect, I think if he would have just stepped down six months early and focused on his defense, things may have turned out differently,” said Mr. Petersen. “Who knows?”