Kellyanne Conway, a senior adviser to President Trump, said in an interview from the White House that people should "go buy Ivanka's stuff."
Conway made the plug Thursday in an interview with Fox News Channel, one day after the president attacked Nordstrom department stores for dropping his daughter's fashion line.
"This is a very successful businesswoman," Conway said, before offering statistics about how many stores sell Ivanka Trump merchandise and calling her a "champion for women empowerment."
"Go buy Ivanka's stuff, is what I would tell you," Conway said. "It's a wonderful line. I own some of it. I fully -- I'm going to just, I'm going to give a free commercial here: Go buy it today, everybody. You can find it online."
Federal law says that public employees may not use their positions "for the endorsement of any product, service or enterprise, or for the private gain of friends, relatives, or persons with whom the employee is affiliated in a nongovernmental capacity."
Larry Noble, the general counsel of the Campaign Legal Center, a nonpartisan organization of election law experts, said that in his opinion, Conway "may have violated the law."
The White House did not immediately respond to a request for comment.
Richard Painter, who served as chief ethics lawyer in the George W. Bush administration, said he would not address whether any White House official was violating the law.
But he said public officials, when giving public speeches or TV interviews in an official capacity, may not promote the products of "a particular private business belonging to the employee's own family, the President's family, a friend, a campaign contributor or anyone else."
"That was strictly forbidden in the Bush administration because it is illegal," he said.
Stan Brand, a former chief legal officer for the House of Representatives, said that Conway's endorsement appeared to be "technically a violation."
He suggested it was doubtful that any law enforcement official would pursue "a single statement like this," but he said "a pattern or practice of such conduct could become a problem."
Nordstrom said last week that it would no longer carry Ivanka Trump's line of clothing and accessories because of "brand performance." An online campaign called #GrabYourWallet has encouraged shoppers to boycott Ivanka Trump merchandise.
In addition, the company that owns TJ Maxx and Marshalls said that it recently sent a memo to workers instructing them not to highlight the Ivanka Trump brand in stores. It did not provide a reason for those instructions.
And the Belk department store chain said it plans to pull Ivanka Trump products from its website, but will continue to offer them in stores. Belk said the decision was a response to customer feedback.
On Wednesday, Trump tore into Nordstrom for mistreating his daughter.
"My daughter Ivanka has been treated so unfairly by @Nordstrom. She is a great person -- always pushing me to do the right thing! Terrible!" he tweeted.
The message was retweeted by the official presidential Twitter account, @POTUS, and raised eyebrows among ethics lawyers.
Noble said the president's tweet was "totally out of line."
"He should not be promoting his daughter's line, he should not be attacking a company that has business dealings with his daughter, and it just shows the massive amount of problems we have with his business holdings and his family's business holdings," Noble said Wednesday.
The rules on endorsements by public officials exempt the president and vice president.
Sean Spicer, the White House press secretary, defended the president's use of the @POTUS handle to discuss his daughter's business.
"This was less about his family's business and an attack on his daughter," he told reporters on Wednesday.