Modern American history has never seen as full-scale an effort to sabotage a valid law as we have with President Trump and the Affordable Care Act — a law whose legality has been upheld twice by the US Supreme Court.
The president has a legal obligation, under Article II of the US Constitution, to "take Care that the laws be faithfully executed." That means he must make sure that our laws are implemented in good faith and that he uses his executive discretion reasonably toward that end.
His agencies likewise have a legal obligation, under the Administrative Procedure Act — the statute that sets the rules for our entire federal regulatory apparatus — not to use their power to engage in arbitrary action.
The intentional, multi-pronged sabotage of the ACA that we have seen over the past nine months — reaching new heights since attempts by Congress to repeal the law failed — violates both Trump's constitutional obligations and quite possibly the obligations of his Department of Health and Human Services.
Trump does not get to say that he can best help the law by killing it and thereby forcing Congress to start afresh. His obligation is to "take care" that the laws that are already on the books are carried out. Since he has flouted this obligation, lawsuits by individuals and states harmed by the damage he causes may now be in order.
To be sure, the take care clause is rarely invoked. Indeed, it does not appear ever to have been used successfully in modern times as an offensive tool against a president. Cases are rare because most experts agree that the president must have discretion with respect to how he enforces a law; drawing lines that separate when that discretion is exercised reasonably and, instead, when it changes too much of the law to be "faithful" to it (as the clause requires) is extremely difficult.
But whatever divides exist about invoking the take care clause, this is the extreme case in which it is clear-cut that the clause has been violated. Far from using his power to faithfully implement the ACA, the president is actively using his power to destroy it. He does not hide his motives.