Mr Trump tweeted that "Ivanka has been treated so unfairly" by clothing retailer Nordstrom.
A Democratic senator called the post "inappropriate" and an ex-White House ethics tsar dubbed it "outrageous".
Earlier this month Nordstrom became the fifth retailer to drop the Ivanka Trump clothing line, citing lack of sales.
The move comes amid a boycott of all Trump products, which activists have dubbed #GrabYourWallet, in reference to a 2005 comment Mr Trump made about women's bodies.
A spokesman for Pennsylvania Senator Bob Casey said the senator "feels it is unethical and inappropriate for the president to lash out at a private company for refusing to enrich his family".
Norm Eisen, who served as ethics tsar under President Barack Obama, called the move "outrageous" and advised Nordstrom to sue Mr Trump under the California Unfair Competition Law.