This goes for Trump too, the president is not there to serve his own interest but those of the millions of American citizens, even those of us who didn't vote for him. Few will believe you ban the entire population of a list of countries arguing that is to make the country safer, when you don't include Saudi Arabia in that list, the country where most of the 9/11 terrorist came from. But when you have business interests with Saudia Arabia, you are using office to push your own personal interests, rather than the interests of all the American citizens.
I assume it is believed to fall under the Emoluments clause that says politicians can't use their position for monetary gain from Foreign governments such as leveraging business contracts. In her case, she is suing for slander because it will harm her future profits, how it's unconstitutional I don't know as First Lady is kind of a made up position and is not a paying position which is one of the requirements for the emoluments clause if I read it right.
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