The ranking Democrat on the House Oversight Committee said Sunday that a counselor to President Trump broke the law last week when she encouraged TV viewers to buy clothing from the fashion brand of Mr. Trump’s daughter.

“This was a textbook case of a violation of the law,” Rep. Elijah Cummings (D) of Maryland said on ABC’s “This Week.” “You cannot go there as an employee of the government and advertise for Ivanka Trump or anyone else.”

The remark by the congressman marked the latest pressure from Capitol Hill to discipline Kellyanne Conway for promoting Ivanka Trump’s brand on Thursday, a comment Ms. Conway made in response to questions about retailers pulling the apparel from their shelves. Mr. Cummings and Rep. Jason Chaffetz (R) of Utah, the chair of the Oversight Committee, also penned a letter to the Office of Government Ethics, urging it recommend the White House take disciplinary action.

Ivanka Trump’s clothing line has pushed Ms. Conway and the previously obscure Office of Government Ethics into the center of an ongoing debate over whether the White House has appropriately distanced itself from the Trump family’s global business empire. White House officials say it has, insisting that Nordstrom and other retailers' actions were a personal attack against Trump’s daughter, and that Conway’s comments are being blown out of proportion. But ethics lawyers and watchdogs, and other critics say the controversy is the latest example of an ongoing problem.

In Conway’s interview with “Fox & Friends” on Thursday, she encouraged viewers to purchase the clothing line.

“Go buy Ivanka’s stuff,” she said. “I’m going to give a free commercial here: Go buy it today, everybody. You can find it online.”

Conway was responding to questions about Nordstrom and other retailers' actions and a boycott of Trump brands, #GrabYourWallet, that appeared online at the same time. Earlier this month, Nordstrom announced, based on sales, it would no longer carry Ivanka Trump’s products, according to The New York Times. The move corresponded with other retailers including T.J. Maxx, Marshalls, and Neiman Marcus giving less prominence in stores to Ivanka Trump’s products.

"I do find it ironic that you've got some executives all over the internet bragging about what they've done to her and her line, and yet they're using the most prominent woman in Donald Trump's...she's his daughter...and they're using her who has been a champion for women empowerment, women in the workplace to get to him," Conway said on “Fox & Friends.”

The Office of Government Ethics forbids government officials from endorsing or promoting products. While the president and vice president are exempt from this federal law, it even extends to participation in fundraising events. In 2014, for instance, the State Department instructed US diplomats not to participate in the ice bucket challenge because it violates internal policy.

"There are firmly established rules preventing the use of public office, such as our ambassadors, for private gain, no matter how worthy the cause,” reads a diplomatic cable sent in 2014, according to NPR.

In response to complaints about Conway, the White House said it has addressed the issue. In a news conference last week, White House press secretary Sean Spicer said that Conway had “been counseled” on the matter, according to CNN. Stephen Miller, a senior adviser to President Trump, added on ABC’s “This Week” that Conway’s comment was a “flippant response” the media has blown out of proportion.

But the comment comes as President Trump also faces accusations from critics that he promoted his daughter’s clothing line through a tweet.

My daughter Ivanka has been treated so unfairly by @Nordstrom. She is a great person -- always pushing me to do the right thing! Terrible! — Donald J. Trump (@realDonaldTrump) February 8, 2017

Trump also shared the tweet through the official presidential Twitter account.

Mr. Spicer defended the president’s use of the @POTUS handle to discuss Ivanka Trump’s business.

"This was less about his family's business and an attack on his daughter," he told reporters on Wednesday.

However, ethics lawyers and watchdogs have sounded the alarm over both Trump and Conway’s actions. They say it’s part of a pattern of the Trump administration not properly separating the president from his past business dealings, as Politico wrote last week.

His Palm Beach club Mar-a-Lago doubled its initiation fee, while Sean Spicer, his press secretary, has labeled it "the Winter White House." The CEO of his hotels business pondered a threefold expansion. Foreign dignitaries are flocking to his Washington hotel. He urged British officials to scuttle a wind farm that would obstruct the view from his golf course. Lawyers for first lady Melania Trump claimed in a New York State libel suit that a Daily Mail article about her cost her the chance “to launch a broad-based commercial brand in multiple product categories, each of which could have garnered multimillion-dollar business relationships."

Congressmen Cummings and Chaffetz have since urged the Office of Government Ethics to step in about Conway, encouraging it to define the appropriate disciplinary actions the White House should take. The congressmen list “reprimand, suspension, demotion, or dismissal” as examples.

But the letter acknowledges that the buck stops with the head of the agency, in this case the president, since the Office of Government Ethics can only recommend actions, not enforce them.

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But Jason Novak, a CNBC columnist notes the letter might send an appropriate message to the administration.

A more likely scenario is that the White House team learns from this experience and self-regulates on matters like this going forward. We're already seeing some forms of moderation from President Trump from his endorsement of the "One China" policy to more circumspect statements about Middle East peace.

Editor's note: The original version of this story misstated Rep. Elijah Cummings's district. It is in Maryland.