Prosecutors laid out a startling corruption case against former Virginia governor Robert F. McDonnell and his wife, but the government faces a high bar in proving that the couple committed a crime, legal experts said Wednesday.

To make its case, the government must show beyond a reasonable doubt that McDonnell (with wife Maureen as co-conspirator) struck a corrupt bargain with a Richmond businessman who lavished $165,000 in cash and gifts on the McDonnell family. They must deliver evidence that the former governor agreed to provide his official help to Star Scientific chief executive Jonnie R. Williams in exchange for his largesse.

“The whole case is going to boil down to proving the quid pro quo,” said Randall Eliason, a George Washington University law professor and an expert on public corruption. “They can’t just show that he got these gifts, but must prove that he received them in exchange for official acts. The defense is going to say, ‘Hey, this is what a Virginia governor is supposed to do — promote Virginia businesses.’ ”

Many legal experts said Wednesday that the coming criminal trial is likely to serve as an important test of the line between political favors and official state action.

The former governor and his wife were charged Tuesday in a 14-count indictment that alleges that they engaged in conspiracy and fraud to use the governor’s office for private gain. The steady stream of gifts the family received — including $15,000 to cater a wedding for one of their daughters, a shopping spree at New York designer boutiques, golf greens fees, vacations and $120,000 in loans — creates a very strong circumstantial case, the experts said.

A simple allegation from a Virginia chef catalyzed a criminal investigation about former Gov. Robert McDonnell. (Jeff Simon/The Washington Post)

“On the side of accepting gifts from a private party while you’re in office, they have a fantastic case,” said Andrew McBride, a defense lawyer who used to prosecute fraud and public corruption in Virginia. “The biggest problem the government has is, what official act did he take as governor to further the interests of Star Scientific?”

The government does not need to show that Williams’s company actually benefited — only that help was promised. But barely a handful of people would know what was discussed among Virginia’s first family at the time and Williams, and two of them — the McDonnells — are charged. They deny committing any crimes.

In the indictment, prosecutors say the former governor hosted a product launch for Star’s dietary supplement, Anatabloc; promoted it at public events; arranged meetings between Williams and senior state health officials; and worked alongside his wife to push for state researchers to consider conducting trials of the product.

Nearly all public-corruption prosecutions are uphill battles, because proving an official’s intent is tricky. Even though ex- Illinois governor Rod Blagojevich was caught on tape trying to sell Barack Obama’s Senate seat, the jury deadlocked on most of the charges in his first trial. In the McDonnells’ case, the prosecution faces other challenges, experts say.

First, much of the government’s case about a corrupt agreement hinges on the testimony of Williams, its star witness. Williams was under investigation for possible securities violations when he decided to cooperate with federal investigators. He obtained immunity in exchange for his testimony, and defense lawyers have said that when he describes on the stand what he and the governor discussed in private conversations, they will seek to show that he is not credible.

“The challenge is, if it comes down to the word of one person, who has real incentive to tell a story the way the government would like to hear it, jurors are likely to be skeptical and focused on those motivations,” said Andrew Wise, a lawyer who represented a defendant in the Jack Abramoff lobbying scandal.

In addition, Maureen McDonnell comes across in the charging papers as the dominant actor — a first lady striving to live the high life and requesting more and more help from Williams. She also was the person most actively pursuing ways the state could promote Williams’s company and get state-funded research for its product, the indictment says.

“The indictment’s a little thin” on what the governor did, Eliason said. “It’s 75 percent about Maureen,” he added. “They’ve got to show she’s doing this with the promise or agreement that the governor would take these special steps.”

But in their charging papers, prosecutors have hinted at some of their strategies for overcoming those challenges — and proving the McDonnells’s criminal intent.

First, they have charged the couple with crimes suggesting that they worked to conceal Williams’s gifts. The former governor is charged with false statements to bank officials for allegedly failing to disclose a Williams loan to a federal credit union. The former first lady is charged with obstructing an investigation by pretending that dresses Williams bought for her were lent to her by Williams’s daughter. Maureen McDonnell returned the dresses with a note thanking her for the loan — days after she first learned that the FBI was investigating her relationship with Williams, according to the charges.

“The coverup stuff is in there in part to show evidence of guilty knowledge,” Eliason said.

“Why do you do that if you aren’t trying to hide some kind of wrongdoing? If it’s all on the up and up, why are you concealing this loan?”

Prosecutors also refer in the indictment to their “stream of value” theory in arguing that the McDonnells were offering their services in exchange for the financial help from Williams.

By showing that numerous gifts were part of a stream the McDonnells received, the government avoids the burden of having to prove an agreement between the parties for each gift.

“This stream-of-value theory allows the government to argue that the public official was essentially on retainer,” Wise said.

Experts said that a wealth of examples can help prosecutors make their case. They can show a series of official steps the McDonnells took to help Williams, at his request — alongside a long stream of gifts and money from him.

“Eventually, you build up enough bricks in that wall that you prove beyond a reasonable doubt that this was what was going on,” Eliason said.

But the law that defines a corrupt agreement is vague, which can present a major problem for the defense, too, Wise said.

A bribery agreement need not be specific or explicit. It can conceivably be as vague as an official saying, “I’ll take care of you on that,” or “Let’s look out for one another.”

The McDonnells accepted so many big-ticket items it would be hard for prosecutors not to pursue the case, said Peter Zeidenberg, a defense lawyer who was part of the team that prosecuted I. Lewis “Scooter” Libby, former vice president Dick Cheney’s chief of staff.

“When you start talking about a $50,000 loan and paying off the kid’s wedding bill, those certainly make you sit up and pay attention,” Zeidenberg said. “I can see why the government moved on this.”

Zeidenberg said prosecutors have brought a “pretty strong case, but it’s not a slam-dunk either way.”

“Because of the McDonnell brand, I think some jurors will come with an open mind that this could have been a misunderstanding, rather than a corrupt plot,” he said.

“He doesn’t have this persona as a sleazy politician.”