The constitution is a brief, vague document. It lays out the powers of the various branches in only the most general terms. The particulars of who actually does what depend much more on decades of habit than on anything written down. From time to time a crisis arises, and Congress passes a law demanding particular Presidential actions -- for example the War Powers Act, passed in the aftermath of Vietnam to define when and how the President may use military force without a declaration of war. But in general these are political decisions, worked out by the politicians among themselves, with occasional input from the voters. Eric Posner explains:
Would President Barack Obama, by refusing to enforce the immigration laws against millions of undocumented immigrants, be engaging in “domestic Caesarism,” as NY Times columnist Ross Douthat charges? . . .Are Obama's invocations of executive authority worse than those of other modern presidents? That is, I submit, a question of taste. I have personally been annoyed by two of his decisions, his refusal to prosecute soldiers and CIA agents who tortured prisoners and his military intervention in Libya. The Libya case is instructive. Under the War Powers Act Obama should (I think) have notified Congress of his actions and asked for their approval, which he did not. (Instead he sent his spokesman out to advance the strange doctrine that this wasn't war because we were only dropping bombs.) But he did not because Congressional leaders of both parties made it clear that they would rather not get involved. If Congress is not going to insist on its own powers, why should the President respect them?
It’s not entirely clear what Obama plans to do, but if he chooses not to enforce immigration laws against “up to half the country’s population of illegal immigrants,” as Douthat claims, the president wouldn’t be doing anything different from what his predecessors have done (or rather, not done). . . .
The executive branch spends a lot of time not enforcing laws. Congress has illegalized an enormous amount of activity without giving the president the resources to enforce the laws, so the executive has no choice but to make a list of priorities and devote its attention to law violations that, in its opinion, are the most serious. Thus, the IRS doesn’t audit paupers very often. The Justice Department ignores a lot of anticompetitive behavior that might raise prices a bit but not much. The DEA focuses on criminal syndicates rather than ordinary drug users, although both violate federal law. And so on.
Nearly all of this non-enforcement takes place with implicit congressional acquiescence; once in a while, Congress complains because the president’s priorities are not the same as its own. But the president has no obligation to listen to these complaints. The Constitution gave him executive power while preventing Congress from compelling the president to act except by issuing the extreme and usually non-credible threat of impeachment. This is the separation of powers. People like Douthat wrongly think that separation of powers means that the president must do what Congress decides. That’s not the principle of separation of powers; that’s the principle of legislative supremacy, embodied in parliamentary systems like Britain’s, which America's founders rejected.
When it comes to illegal immigrants, here's my question: has any President ever taken these laws seriously? There are around 12 million undocumented immigrants in the country, and the great majority arrived before Obama was inaugurated. Why are they suddenly his fault? It would not be impossible to force many of them out, but this would require unpopular measures like biometric ID cards or crackdowns on contractors who hire undocumented day laborers. Americans don't want the level of government interference in economic life it would take to really root out the undocumented, nor do we want to pay higher taxes to fund a hundred thousand more federal agents. So what is the President supposed to do? His only choices are to let the system run on as it has in a more or less random way, or set priorities himself. By explicitly setting priorities for immigration agents, Obama is only carrying out his constitutionally-appointed duties.