The standards chief of the US Congress says a senior Trump aide was "wrong, wrong, wrong" to promote Ivanka Trump products on live television.
Jason Chaffetz, a Republican who heads the oversight committee in Congress, said the promotion was "clearly over the line, unacceptable".Trump aide Kellyanne Conway had said on Fox News: "Go buy Ivanka's stuff."
Her comment came after retailer Nordstrom dropped Ivanka's clothing line, citing a lack of sales.
In response, President Donald Trump tweeted to say his daughter had been treated "so unfairly" by the retailer.
The White House said on Thursday that the president "absolutely" continued to support Ms Conway, despite intense criticism of her remark by politicians.
Federal ethics rules prevent White House employees giving an "endorsement of any product, service or enterprise".
Mr Chaffetz said the White House should notify the Office of Government Ethics so it could initiate an inquiry. There have been numerous concerns over possible conflicts of interest for White House staff.
He said he and his Democratic counterpart would write to Mr Trump.
"It needs to be dealt with," he told the Associated Press. "There's no ifs, ands or buts about it."
White House spokesman Sean Spicer said Ms Conway had been "counselled" after her remarks.
But the calls for further investigation or sanctions are growing. "It's a violation of the rule," Norman Eisen, a former ethics adviser to Barack Obama, told MSNBC. "It's a serious matter."
Two liberal lobby groups have filed complaints with the Office of Government Ethics.
"Conway's action reflects an ongoing careless disregard of the conflicts of interest laws and regulations by some members of the Trump family and Trump administration," Craig Holman, government affairs lobbyist for one group, Public Citizen, told AP.