Prof. Somin, like Prof. Obama, argues that the President is given very broad discretion in enforcing the immigration laws, and presidents necessarily choose which laws to enforce, so the decision to rewrite federal immigration laws by refusing to enforce them is legal. I don't think so.
The Constitution says that the President "shall take care that the laws be faithfully executed." Note how mandatory that language is. Not that he "should" or "will" enforce the law; he shall. And he's not just to enforce the law; he's to take care about it. And he isn't even just to enforce the law--he's to execute the law. Most of all, he is to do so faithfully. As in, in good faith. As in, in their letter and spirit. If you were writing a Constitution, and wanted to write language that would, as clearly as possible, forbid the president from choosing which laws to enforce and which not to, just to suit his own personal desires, this language is exactly what you would choose.
A president faced with limited resources who chooses to prosecute only the severest crimes the budget will allow, is faithfully taking care that the laws be executed. He's doing his best to see to it that the laws are enforced "to the best of [his] ability." Obviously choices must be made, and when made, will be made on accordance with policy. Nobody argues that the President has an "absolute duty to prosecute all violations of federal law." But he does have a duty--as absolute as any the Constitution contemplates--to take care that the laws be faithfully executed. And breaking the law--taking actions the President himself has repeatedly admitted are illegal--not as a result of budgetary problems, but simply because he disagrees with current law, is not faithful execution of the law. In fact, it is just about as clearly the opposite of that as you could get.
That's the import of the word "faithfully." In a world of limited resources, a president who says, "Well, I don't like the law, so I'm just going to use my resources to enforce other laws," can always claim he's just using his discretion. But that would not be faithful execution. Remember that the Clause in the Constitution that limits "prosecutorial discretion" is the Take Care Clause. The Constitution does not contemplate the president being legislator-in-chief. His job is to execute the law, and to do so faithfully. Prof. Somin writes that there's no distinction "between case-by-case decisions not to prosecute...and a generalized, systematic policy of not doing so in a category of cases." But this is just the fallacy of the beard. Obviously there won't be a bright line in such cases; the idea of "faithful" is too broad for that. But that doesn't mean we can't see an obvious violation for what it is. Here, the violation obviously falls on the other side. A systematic plan to disregard the law, not because it's unconstitutional, but simply because the president disagrees with the law and would prefer the law be different, is obviously not taking care that the laws be faithfully executed. Prof. Somin says we can't read the clause so strictly that "every president in the last century or more would be in violation"--and he's right; and we don't--but he reads the clause so broadly that no president would ever be in violation.
Something has to violate the Take Care Clause. If "a generalized, systematic policy of not [enforcing the law] in a category of cases" just because he doesn't like the law isn't a violation, what would be?
Now, there is one thing that I think the President could do here that would have exactly the same result as the President is seeking, and which is perfectly constitutional: the pardon power. The President is allowed unlimited discretion to pardon violations of the law, with no effective check or balance by the other branches. That's an executive power the Constitution doesn't limit (unlike the Take Care Clause, whose sole purpose is to limit "prosecutorial discretion"). Pres. Obama could pardon illegals all he wants. But--as is typical of this administration--he's chosen to go out of his way to choose the unconstitutional route, for political reasons, rather than the route our supreme law provides.
Of course, I don't expect any of this to matter. The Republican Congress has never really tried to rein in Presidential overreach. Indeed, as Prof. Somin rightly argues, Congress is largely at fault for it, by passing laws that give enormous discretion to the executive branch. But as far as the Constitution is concerned, Pres. Obama's decision to disregard the laws because he doesn't like them cannot be rationalized as faithful execution of the laws.
Update: Seemes to me Prof. Somin gives up when he says that the President would violate the Take Care Clause if he "chose targets for prosecution not based on the severity or importance of the crimes they committed but based on personal or political animus towards the accused." I see no difference in principle between that and choosing not to enforce the law out of political animus toward the law itself. Both are bad faith refusals to see that the laws be executed, and thus violate the Constitution. The only alternative to that view, it seems to me, is to read the word "faithfully" out of the Clause and thus allow the President unlimited authority to choose which laws to enforce and which to disregard, based on his own preferences. And that is the one thing the Clause plainly forbids.
Update 2: I won't belabor the point except to say that Prof. Somin just keeps begging the question by saying that the President violates the Take Care Clause when he acts in ways that are "not...law enforcement objective[s] at all, but a matter of personal antagonism between the executive and the defendant"--since the refusal to prosecute for similarly non-enforcement objectives is identically unconstitutional. Non-enforcement out of personal approval of the defendant. Or his conduct is just the other side of the same unconstitutional coin. Prof. Somin can't say that one is unconstitutional without admitting the other is, too.