After Virginia Gov. Robert F. McDonnell (R) endorsed Mitt Romney for president in 2012, McDonnell’s wife sought out the candidate to promote the dietary supplement now at the heart of the former first couple’s corruption trial, a onetime aide testified Monday.

Maureen McDonnell and then-Star Scientific chief executive Jonnie R. Williams Sr. showed up together at a news media session in South Carolina hoping to pitch Anatabloc to the prominent Republican, said Phil Cox, Robert McDonnell’s chief political adviser at the time.

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Cox, also executive director of the Republican Governors Association, said that he put a stop to that plan but that Maureen McDonnell went on to talk up the supplement to Romney’s wife on a campaign bus. He said she told Ann Romney that the anti-­inflammatory supplement, Anatabloc, could “potentially cure MS.”

While Ann Romney, who has multiple sclerosis, listened politely, Cox said, he feared the episode would reflect poorly on his boss, who at the time was considered a possible Romney running mate.

“I was horrified,” Cox testified. “I thought it was a train wreck.”

The sixth day of the McDonnells’ trial — in which they are accused of using the governor’s office to benefit Williams in exchange for his lavishing vacations, luxury goods and cash on them — was marked again by revelations that could be damaging to the couple. It was also notable in that Williams — after 15 hours on the witness stand — finally stepped down with his account largely intact and with a few key points clarified in prosecutors’ favor.

Williams testified Monday that the photo he was texted in December 2012 showing Robert McDonnell grinning and flashing a Rolex watch came from the governor. That is important because McDonnell has said publicly that he received the expensive timepiece as a Christmas gift from his wife a year earlier.

Williams said the picture was a confirmation to him that McDonnell knew he had purchased the watch.

While the sender is disputed — defense attorneys note phone records show no corresponding text message; prosecutors say iPhones sometimes do not keep such records — Williams was confident in his assessment. Claiming “it said G-O-V on my phone” when the message came, Williams testified that the image was as troubling as it was unforgettable.

“This shows up on the nightly news, I have a problem,” Williams testified.

Williams also seemed to score points when defense attorney Henry Asbill questioned him about a phone call the governor made to Williams’s father on his 80th birthday. The inquiry seemed to be designed to show Williams and the governor had a personal relationship. Williams had other ideas.

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)

“This was a very personal thing for my father,” Williams said. “But this cost me, in my mind, as I sit here, hundreds of thousands of dollars to even do that.”

Williams is far from a rock- solid witness, and defense attorneys will continue to attack his credibility as they question others in the case. Even Cox — whose testimony immediately followed Williams’s — called the businessman “a little bit of a snake oil salesman” who “seemed like he was trying a little too hard.”

But Cox’s criticisms were not reserved for prosecutors’ star witness. He said, for example, that he told the governor that he was angry and frustrated and that he thought McDonnell had exercised poor judgement after learning from a Washington Post story that Williams had lent $120,000 to Maureen McDonnell and a small real estate company owned by the governor and his sister. However, Cox, who served as McDonnell’s campaign manager and top political adviser, said he still considers the governor a friend.

Cox also took aim at Maureen McDonnell. Of particular note: Cox said he was involved in rejecting Williams’s offer in 2009 to buy an Oscar de la Renta dress for her to wear at her husband’s inauguration. The first lady, he said, then dashed an e-mail off to him — on Christmas Eve — complaining that he was not loyal to her and the governor.

“The e-mail literally had no basis in reality,” Cox testified, adding that he’d been concerned about the “optics” of the first lady wearing an expensive gown in the height of a recession. “It was sort of an insane rant of an e-mail, and coming on Christmas Eve, it angered me.”

Cox also testified that Maureen McDonnell “adored” Williams and “would light up when he walked into a room” and vice versa. E-mails shown to jurors later at the trial seemed to press the point. When the governor’s scheduler Monica Block was asked to shoehorn a Williams-driven event into her boss’s calendar, she asked another McDonnell aide (and future son-in-law) in an e-mail whether that was necessary.

“Yes. 102 loves Johnnie,” the aide replied, using the state police’s numerical code for the first lady and misspelling the executive’s name.

“Why does 102 like him? Because he’s loaded?” Block responded.

“Yep. He wanted to make the gown for the inauguration,” the aide answered.

Those are important sentiments because William Burck, Maureen McDonnell’s defense attorney, has asserted that his client’s marriage was in shambles — making it impossible for her and her husband to conspire together, as prosecutors must prove they did. Burck has said his client was innocently seeking Williams’s affection, rather than corruptly seeking his money.

How jurors view the relationship between the two could make or break the case.

Assistant U.S. Attorney Michael Dry launched a vigorous attack Monday on the notion that Williams and Maureen McDonnell had any sort of romantic connection — flashing pictures of the first lady and her husband whispering or holding hands as Williams flatly denied any sort of tryst with the governor’s wife.

Williams denied any “physical contact” with the first lady and said neither of them had ever suggested romantic interest to the other.

As if to drive home the point, Dry asked about Williams’s attendance at the McDonnells’ anniversary party and about items — the Rolex and a Ferrari to use on a vacation — that Maureen McDonnell is accused of soliciting for her husband.

“Who does she want that car available for?” Dry asked.

“Her husband,” Williams responded.

“When Mrs. McDonnell asked you to buy the Rolex, who was the Rolex going to?” he continued.

“Her husband,” Williams said.

Justin Jouvenal contributed to this report.