Critics say the US president has misused his office by supporting a family business.
US President Donald Trump once again prompted a controversy on Wednesday when he criticised department store chain Nordstrom for stopping sales of his daughter Ivanka's clothing brand.
Trump used his personal and official twitter accounts to say that the store has treated his daughter unfairly.
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
The wealthy New York real estate developer who became president on January 20 has attacked businesses in the past as part of his election campaign.
But this is first time he blasted a firm involved with his family's business, raising conflict-of-interest concerns.
Nordstrom said its decision was based on declining sales of Ivanka Trump products that include clothes, shoes and accessories.
The move to remove Ivanka's products comes amid an ongoing campaign called #GrabYourWallet, which encourages shoppers to boycott products with ties to Trump, his family and his donors.
White House spokesman Sean Spicer characterised Nordstrom's decision as a "direct attack" on the president's policies.
But critics accuse Trump of abusing his position.
"This is misuse of public office for private gains," Richard Painter, who served as Republican president George W Bush's chief ethics lawyer, said in an email to Reuters.
"And it is abuse of power because the official message is clear - Nordstrom is persona non grata with the administration."
Nordstrom could have cause for legal action if the Trump administration takes any adverse action against it, he said.
Trump has in past favoured some companies while attacking others.
Earlier this month he encouraged people to shop at L.L.Bean, the online retail company that donated to his campaign.
Thank you to Linda Bean of L.L.Bean for your great support and courage. People will support you even more now. Buy L.L.Bean. @LBPerfectMaine— Donald J. Trump (@realDonaldTrump) January 12, 2017
He has also targeted Boeing, Carrier and Lockheed Martin in the past.
Democrats pounced on Trump for the tweet.
"I think it's inappropriate, but he's a totally inappropriate president, so it's totally in keeping with who he is," said Nancy Pelosi, the top US House of Representatives Democrat.
Senator Bob Casey, a Democrat, in a tweet indicated the matter should be referred to the federal Office of Government Ethics.
"President Trump said he is going to have nothing to do with his family businesses," said Robert Weissman, president of liberal watchdog group Public Citizen.
"His reaction to developments with his daughter's business line suggests that claim is untrue."
Trump's tweet left fellow Republicans in an uncomfortable position.
Republican Senator David Perdue of Georgia told Reuters of Trump's tweet, "That sounds like a personal matter to me." Perdue added, "He is a citizen; and he is a citizen who is now president of the United States."
Republican Senator Ron Johnson of Wisconsin, asked whether the president should be criticising a publicly traded company over its business dealings with Trump's daughter.
On Tuesday, First Lady Melania Trump refiled a $150 million defamation lawsuit accusing the Daily Mail British tabloid of damaging her brand by reporting rumours she had worked as an escort in the 1990s.