I guess legal expert Clem doesn't know that the Attorney General of the United States swears an oath to support and defend the Constitution of the United States, not an oath of loyalty to the president.
An AG (acting or not) is not obligated to enforce an order if they believe it is unconstitutional or illegal.
They are supposed to be independent.
Yates pointed out today to Senator John Cornyn (R- Texas) that during her confirmation hearing in 2015, when Obama appointed her deputy AG, he specifically asked her if the president asked her to do something that was unlawful or unconstitutional, would she say no, and she promised that she would that.
Jeff Sessions asked her the same thing, and she replied the same way.
BTW Jeff Sessions was asked the same question in his confirmation hearing for AG, and he gave the same answer, because that is the right answer.