From my Politico column today:

The president and his supporters pretend that the Immigration and Nationality Act contains a gigantic asterisk that says, notwithstanding the elaborate legal infrastructure set out in the law and the distinctions among different categories of immigrants, the president can do whatever he wants.
No Congress would ever write the law this way. And even if it did, it wouldn’t pass constitutional muster.
“The case law,” according to David Rivkin of the law firm Baker Hostetler, “clearly recognizes that delegations of any type of legislative authority to the president must contain some limiting principles; they can never be open-ended. To do otherwise, would unconstitutionally transfer core legislative powers to the president.”
The president’s defenders rely on the notion of prosecutorial discretion, the existence of which is uncontroversial. The executive doesn’t have the resources to hunt down and prosecute every violator of our laws, and therefore has to establish enforcement priorities.
The Congressional Research Service did a report on prosecutorial discretion and immigration that, for the most part, emphasizes its piddling reach. It says, for instance, that immigration officers may use discretion to decide whom to stop, question, and arrest; whether to issue or cancel a Notice to Appear; whether to settle or dismiss a proceeding; and so on.
No one heretofore has thought this leeway could be used by a president as warrant to eviscerate an entire statutory scheme.