Washington – Donald Trump on Wednesday lashed out at department store chain Nordstrom for dropping his daughter's clothing line, again spotlighting the intermingling of the US presidency with Trump family businesses.The public rebuke, which the White House later defended, called renewed attention to the potential tangle of business interests Trump brought with him on taking office last month.In a tweet posted moments after he wrapped up an address to US law enforcement, Trump hit out at the high-end retailer for announcing last week it had decided to discontinue sales of Ivanka Trump's fashion line due to poor sales."My daughter Ivanka has been treated so unfairly by Nordstrom," Trump wrote. "She is a great person – always pushing me to do the right thing! Terrible!"Since his surprise victory in the November presidential election, Trump has used his Twitter feed to lambast individual companies – from General Motors to Boeing – be it for off-shoring jobs or allegedly overcharging the federal government for aircraft.Family businessBut the latest tweet was different in that it sought to defend part of Trump's family business empire, which critics have said could be a source of profound conflicts of interest for the White House.Trump made sure to give his message on Ivanka maximum reach by posting it both on his personal handle @realDonaldTrump and on the official account of the US presidency @POTUS.Since his November victory, Trump has touted an effort to remove himself from running his business empire, transferring corporate control to his sons. But he has resisted divesting, as a government ethics watchdog had called on him to do.Critics say the Trump businesses still pose a significant ethical quandary. 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 Further playing into the running debate, Pentagon officials said on Wednesday they were looking to rent space in Trump Tower, Trump's flagship Manhattan luxury building, to accommodate equipment and staff who accompany the president during his stays there.That came on the heels of a lawsuit filed by Melania Trump in New York, which claimed that damaging rumours reported by a British tabloid had interfered with her "unique, once-in-a-lifetime opportunity" to earn millions of dollars due to her raised profile as first lady.Boycott callsWhite House Press Secretary Sean Spicer on Wednesday defended Trump's Nordstrom tweet, saying the president was standing up for a family member."There's clearly efforts to undermine that name based on her father's issues or particular policies," Spicer told reporters."For someone to take out their concern of the policies against a family member of his is simply not acceptable and he has every right to speak out about it."Nordstrom responded on Wednesday, reiterating that its decision to drop the Ivanka Trump line was made purely on business grounds."Over the past year, and particularly in the last half of 2016, sales of the brand have steadily declined to the point where it didn't make good business sense for us to continue with the line for now," the company said in a statement to AFP, adding that it had "a great relationship" with Ivanka Trump's business.Nordstrom is one of several US businesses that has faced boycott calls for its association with the Trump brand. It is among the firms targeted in a "Grab Your Wallet" campaign launched by anti-Trump activists in protest at the Republican billionaire's agenda.The campaign on Wednesday was still targeting other retailers such as Macy's, Bloomingdale's and Dillard's for carrying Ivanka Trump products.Others such as PepsiCo and Budweiser have faced a backlash from the opposite camp after moves deemed critical of the Trump administration, signs of the fine line that firms are treading in the current political era.Ethical obligationsRichard Briffault, an expert in government ethics at Columbia Law School, told AFP that Trump's use of the presidential bully pulpit to defend his daughter's business "was inconsistent with any notion of the ethical obligations of a public official"."What this suggests is that he hasn't fully internalised the consequence of being the most important public official in the country," Briffault said.Since a 1989 executive order, federal officials have been barred from using public office for private gain, Briffault said, adding that any public criticism from a sitting president could be interpreted as an attempt to influence that company's business decisions."It gives the appearance that he is using his position to promote the business interests of a close relative," said Briffault.Unlike other companies he has attacked on Twitter, including Lockheed, Boeing and Ford, which saw their share prices suffer following criticism from Trump, Nordstrom's stock finished up by more than 4.0% on Wednesday.The news of Trump's remarks preceded a report from The New York Times on Wednesday, which said TJX Companies, the parent of clothing retailers TJ Maxx and Marshalls, had told employees to discard all Ivanka Trump promotional signs and not to display her clothing separately.