Conway and officials from the White House and the Office of Government Ethics did not respond to requests for comment Thursday.
Attorneys, including Campaign Legal Centre general counsel Lawrence Noble, said Conway's endorsement directly conflicted with OGE rules designed to separate government policy from private business dealings.
Don Fox, former general counsel and former acting director of OGE, told The Washington Post that "Conway's encouragement to buy Ivanka's stuff would seem to be a clear violation of rules prohibiting misuse of public office for anyone's private gain."
He added: "This is jaw-dropping to me. This rule has been promulgated by the federal Office of Government Ethics as part of the Standards of Conduct for all executive branch employees and it applies to all members of the armed forces as well."
Enforcement measures are largely left to the head of the federal agency - in Conway's case, the White House. One lawyer said a typical executive-branch employee who violated the rule could face significant disciplinary action, including a multi-day suspension and loss of pay.
Conway's endorsement comes as the Trump administration faces growing scrutiny over whether it is taking fears of conflicts of interest seriously.
The president took to Twitter on Wednesday to lash out at Nordstrom for dropping Ivanka Trump's line, saying his daughter had "been treated so unfairly" by the store.
Said Peter Schweizer, who has worked closely with Trump aide Stephen Bannon and wrote the book Clinton Cash, which was critical of donations to the Clinton Foundation: "They've crossed a very, very important bright line, and it's not good. To encourage Americans to buy goods from companies owned by the first family is totally out of bounds and needs to stop.
"Clearly, the Trumps feel some of this is related to politics. But whether that's true or not, these marketing battles need to be fought by Ivanka and her company. They cannot and should not be fought by government employees and the White House," Schweizer said. "It's time to move beyond the mind-set and the role of a businessman and assume the mantle of commander of chief."
Conway's endorsement of the Ivanka business also highlights an awkward reality for a White House threatening US companies seeking to move jobs or operations overseas. Nearly all of Ivanka-brand merchandise is manufactured in low-cost labour countries, including China, Indonesia, and Vietnam.
Trump critics quickly seized on the endorsement. Robert Weissman, president of liberal advocacy group Public Citizen, said in a statement, "Conway's self-proclaimed advertisement for the Ivanka Trump fashion line demonstrates again what anyone with common sense already knew: President Trump and the Trump administration will use the government apparatus to advance the interests of the family businesses."
Trump last month tweeted his own support for another retailer, L. L. Bean, saying, "People will support you even more now. Buy L.L.Bean." A company board member, Linda Bean, donated money to a pro-Trump super PAC. The federal ban on endorsements specifically exempts the president and vice president.