Jonnie R. Williams Sr. said he had done wrong.

The wealthy Richmond-area businessman took the witness stand last week and told jurors that he had bribed former Virginia governor Robert F. McDonnell (R) and his wife. By Williams’s account, he got them to promote his dietary supplement in exchange for Ferrari rides and luxury vacations, designer clothes and five-figure loans, a Rolex and $5,000 cognac.

But more than mere penitent, Williams was first and foremost a salesman.

Even as the former Star Scientific executive said he’d been wrong to shower gifts on the McDonnells, he spoke in glowing terms about the tobacco-based pill he’d invented. He said it would revolutionize medicine as antibiotics once had. That he’d used it to cure his wife’s serious thyroid disorder. And that he’d discovered a way to remove carcinogens from tobacco using his home microwave.

A one-time car salesman with no medical or science degree, Williams, 59, dropped a prestigious name, Johns Hopkins endocrinologist and Star consultant Paul Ladenson, to back up claims about his new supplement.

Prosecutors in the corruption case against former Virginia Gov. Robert McDonnell and his wife, Maureen, introduced the infamous Rolex watch on Thursday as a key piece of evidence in the trial. The Post’s Rosalind Helderman explains. (Jackie Kucinich/The Washington Post)

“Dr. Ladenson thinks the anatabine discovery is one of the most significant discoveries in medicine, right up there with antibiotics,” Williams told the jury, referring to the active ingredient in his supplement, Anatabloc.

In a case that for the first time in Virginia puts a former governor on trial, the government’s star witness — whose testimony of a conspiracy is key — is an irrepressible pitchman. And legal experts say that could cut both ways weeks from now, when the jury begins weighing the McDonnells’ guilt or innocence.

Tanned, relaxed and sometimes witty on the stand, Williams might charm and convince the eight men and four women who hold the former first couple’s fate in their hands. But there’s also a risk that jurors will see him as a snake oil salesman, one who duped the McDonnells and is lying now to save his own skin.

So far Williams has testified for just short of 12 hours, apparently on occasion testing the patience of both sides. At one point, Assistant U.S. Attorney Michael S. Dry, soliciting wishy-washy answers, told his witness to “stop beating around the bush.” Under grilling from defense attorneys, Williams has repeatedly professed that he could not remember the details of various encounters, especially those with law enforcement.

Defense attorneys have tried to convey to jurors that Williams has given conflicting stories about whether his gifts were explicitly tied to promised help from the McDonnells. They argue that he is testifying only because he has immunity from a laundry list of possible charges, including what defense attorneys have characterized as a $10 million stock fraud.

But before trying to poke holes in Williams’s story, the defense invited Williams to puff himself up.

“Tell us about yourself,” said William Burck, Maureen McDonnell’s attorney.

That seeming softball opening to Burck’s cross-examination launched a 90-minute soliloquy by Williams. At one point, he asked the lawyer whether he was saying too much.

“If you want me to stop, just say so,” he said.

But Burck was all ears.

“I thought it was a very interesting and well-thought-out technique that Mrs. McDonnell’s attorney used,” said Ted Kang, a lawyer at Alston & Bird who once worked in the Justice Department’s public integrity unit.

“Giving him enough rope to hang himself on can accomplish at least two things,” Kang said. “Number one, that this guy is a master salesman. He can sell anything, including a product that he put in a microwave that cures cancer. So how can you believe, ladies and gentlemen of the jury, that what he’s telling you . . . can be true? Number two, Jonnie is not only playing you, the jurors. He is also playing the McDonnells.”

The government is not hinging its case solely on the testimony of Williams, who is set to be back on the stand Monday. As Dry questioned the former Star chief about gifts and loans, the prosecutor introduced exhibit after exhibit to back him up.

Invoices, e-mails, contracts and checks confirmed yacht rentals, luxury vacations, wedding catering, $120,000 in loans and a Bergdorf Goodman shopping spree. A photo of the governor, smiling and sporting sunglasses, documents his spin behind the wheel of Williams’s Ferrari. The Rolex that Williams bought for the governor was not only introduced into evidence — Government Exhibit 33 — but was also handed to jurors to hold and read the inscription: “71st Governor of Virginia.”

But the crux of the corruption case is not whether the McDonnells accepted gifts from a businessman seeking to benefit from their connections. Nor is it enough to show that the McDonnells helped Williams by promoting Anatabloc at conferences, as the first lady did, or giving him an audience with top state officials, as the governor did.

Prosecutors must prove that the couple agreed to help Williams in exchange for those gifts.

The McDonnells contend that what they did for Williams was in line with their efforts to boost other Virginia-based enterprises, such as the wine industry. But Williams testified that Maureen McDonnell had told him that they were broke and that if he helped them financially, she would help him, with her husband’s blessing. Williams said that soon after that exchange, he lent Maureen McDonnell $50,000 and wrote a $15,000 check to cater the wedding of one of the McDonnells’ daughters. He said the governor thanked him but did not discuss any quid pro quo.

Williams testified that he spelled out for the governor exactly what he wanted for his company during an October 2010 plane ride. But he said McDonnell simply referred him to a state health official, and Williams acknowledged later that he might have gotten a meeting with that official without providing any gifts.

If the jury finds Williams’s stories suspect, prosecutors have something else on their side: timing. Much of the help the McDonnells provided to Williams came right about the time the businessman opened his checkbook, vacation home or Learjet for them.

Maureen McDonnell pitched Anatabloc before a group of investors in Florida just three days before her daughter celebrated her wedding with $15,000 in catering from Williams. Robert McDonnell directed his health secretary to send a top deputy to meet with Williams for an Anatabloc pitch the night the governor returned — in the executive’s Ferrari — from a family vacation in Williams’s lakeside home.

The governor e-mailed an aide to ask for a meeting about the university studies sought by Williams six minutes after e-mailing the executive to discuss a loan.

And memory lapses and inconsistencies aside, Williams had some winning moments on the stand, with one-liners that occasionally prompted laughter.

When asked whether he had any instructions about the Ferrari he lent the McDonnells, he said: “I asked her not to let the children drive it.”

Even Robert McDonnell cracked a smile at that and again when a prosecutor asked Williams about the Louis XIII de Remy Martin cognac he had served the governor and others.

What kind of cognac was that?

“Expensive cognac,” Williams deadpanned.

But his recitation of his life story wore thin after a while, particularly as the defense allowed him to talk on and on about his supplement.

A man in the back row of the courthouse appeared to drift off, as Robert McDonnell had in 2010, when Williams pitched the governor on Anatabloc for the length of a cross-country flight in the executive’s jet.

U.S. District Judge James R. Spencer declared that he had heard enough for one day.

“We’re going to stop right here,” he said, interrupting the questioning at 5:30 p.m. sharp, “primarily because I can’t take another second.”