This is the same ploy as the "let's ban burning the flag" bit - he knows it can't work that way, but he says it anyway to highlight the common-sense arguments.
Trump will get hard pushback from this and the watching public will emphasize the need for stronger enforcement of existing law instead of this nonsense.
>Mr. President, it would make more sense to approach this the way you're approaching immigration - the laws are already on the books, they just need to be enforced.
Don't expect this to carry much weight; wait for the counter-claims and watch Trump carry those forward instead.