Prosecutors on Thursday unveiled what could be a critical new piece of evidence in their case against former Virginia governor Robert F. McDonnell and his wife, Maureen: a photograph of the governor, grinning and holding up his wrist to display a watch.

Testifying during the McDonnells’ federal corruption trial, businessman Jonnie R. Williams Sr. said he received the photo by text message in December 2012 in response to one he sent the governor. The watch on McDonnell’s wrist appeared to be the Rolex that Williams had purchased for the governor at the first lady’s request a year earlier.

The picture could shatter any assertion that the governor was unaware that Williams — who was then the chief executive of a dietary-supplement company — had provided the expensive timepiece. McDonnell (R) has previously said the watch was a Christmas gift from his wife.

Many of Assistant U.S. Attorney Michael Dry’s questions for Williams appeared designed to prove that Robert McDonnell was in the loop about what the businessman wanted from the first couple — and was aware of many of the gifts Williams gave them to curry favor.

Williams, who testified for nearly six hours on the fourth day of trial, is the prosecution’s most important witness in the case, which has shined an uncomfortable spotlight on the McDonnells’ marriage and lifestyle.

The businessman repeatedly told jurors that he did not consider himself personal friends with the McDonnells and that he believed the three were in a corrupt business relationship: He would provide luxury gifts and money to help the governor through a difficult financial time, and, in exchange, the couple would help promote a supplement created by his company, Star Scientific.

“I thought the governor could help bring this product to the marketplace, and it was not the right thing to do,” Williams said on the stand. “I knew it was wrong. I thought the ends justified the means, and I was wrong.”

The two sides

To win a conviction, prosecutors have to prove that the former governor and first lady conspired to sell the power of McDonnell’s office to Williams in exchange for such things as private plane rides, luxury vacations, designer clothes and $120,000 in loans.

In essence, they must persuade jurors to believe the account of a wealthy man who bragged of rich and powerful friends and once claimed to have invented a way to remove a cancerous substance from tobacco with his kitchen microwave.

Defense attorneys have called Williams — who has been given immunity in exchange for his testimony — a “master manipulator” and are set to launch a vigorous attack on his version of events.

Williams acknowledged Thursday that when first confronted about his relationship with Virginia’s first couple in January 2013, he told authorities that he had never asked the McDonnells for anything in exchange for his largesse.

Now he said that was a lie. “If he was in trouble, I could be also,” he said he concluded at the time.

A manicurist and a model are just two of the witnesses that may be called to testify in the federal corruption trial of former Virginia governor Robert F. McDonnell and his wife, Maureen, which commenced on Monday. The Washington Post's Matt Zapotosky reports from Richmond on some of the standout witnesses on the list released by prosecutors and defense attorneys. (Ashleigh Joplin/The Washington Post)

Defense attorneys have asserted that the McDonnells’ marriage was crumbling, the two were barely speaking and they could not have conspired to do anything for Williams, a man on whom Maureen McDonnell had a crush. In any case, they have said, Robert McDonnell never promised to perform official acts to assist Williams.

But Williams — who denied any romantic connection to Maureen McDonnell — testified that he laid out his plan for the governor during a five-to-six-hour flight to Richmond from California on his private plane in October 2010, before he gave the first family most of the gifts.

On the flight, he said, he explained that his company’s new dietary supplement needed credibility and that he wanted the governor’s help getting public universities to conduct the studies that would provide it.

Williams said he and the governor discussed using money from the state tobacco commission to fund the project. “He thought it was a good idea,” Williams said. (He did say the governor slept for part of the flight, joking, “I think I put him to sleep.”)

When Maureen McDonnell flew to Michigan in 2011 to address a meeting with doctors whom Star Scientific was trying to impress — one of a number of Star events she attended — Williams said he called the governor the night before to confirm that she had his permission to attend.

Dry showed jurors a video from that event, during which Maureen McDonnell told the crowd that she and her husband were excited about the company.

Later, Williams testified that he held a one-on-one meeting with Robert McDonnell in the governor’s office suite to discuss a $50,000 loan the governor was seeking to help make payments on beach properties the McDonnells owned in Virginia Beach.

Williams said the two discussed a complicated scheme in which the McDonnells would borrow money against Star Scientific stock owned by Williams, an arrangement he hoped would avoid federal and state disclosure laws.

“I said that I’d just as soon keep this between us and no one know this,” Williams testified he told the governor.

Williams said McDonnell replied, “That’s fine.”

Ultimately, Williams said, he simply wrote a $50,000 check to a real estate company owned by the governor and his sister.

The Rolex

Some of the day’s most dramatic moments revolved around the Rolex watch, which Dry dropped on the floor before it was handed to the jurors. They passed it slowly to one another.

Williams testified that Maureen McDonnell asked him to buy the watch, which cost between $6,000 and $7,000, on the day she and Williams held a series of meetings with a state health official and a university scientist.

Williams said the photo of McDonnell wearing the Rolex arrived on his cellphone after he text-messaged a photograph to McDonnell of the governor’s mansion’s executive chef at his house for Christmas Eve.

A forensic analysis has been unable to determine whether the picture was sent to Williams’s phone from the governor’s or his wife’s phone, according to two people familiar with the evidence.

In an initially soft cross-examination of Williams, Maureen McDonnell’s lead defense attorney, William Burck, seemed to want to show jurors that the businessman was a braggart who was not shy about making outlandish assertions (though he was shy about disclosing his net worth). Allowing Williams to describe his own life story at length, he was somewhat successful.

Williams eventually testified that he had discovered how to remove the “strongest and most abundant carcinogens in cigarette smoke” with a conventional microwave oven and then tried to make greater use of the discovery with 200 microwaves he sent an employee to buy from Wal-Mart.

Though Williams insisted throughout the day that his relationship with the McDonnells was a business one, he said he felt a moment of compassion toward Maureen McDonnell when he found a box at his home in March 2013.

By then, he said, he had been interviewed by authorities and knew the first lady had been as well.

In the box were many of the items he had purchased for the first lady on a New York shopping trip nearly two years earlier, along with a note saying that she was returning the clothes as, she claimed, the two had discussed. He said he took the letter to his living room and read it again and again with a “sinking feeling.”

“I felt sorry for her,” he said.

Then, he said, he called his lawyer.

Justin Jouvenal and Rachel Weiner contributed to this report.