I have yet to see any description of Obama's "rock solid legal argument", as the learned folks at the New York Times editorial page describe DAPA, so I thought I would create one now that I am running into it in other cases.
You may have noticed that the Constitution confers all powers over immigration to the Congress. You might then expect that the way the immigration system is supposed to work is that Congress defines the classes of aliens eligible work in the United States and that DHS has the broad authority to determine the individual aliens within those classes who are authorized to work.
That is not how Obama's DHS sees things.
Under their view, DHS and Congress share "dual authority" to define classes of aliens that may work. According to Obama, Congress gave him that authority in the Immigration Reform and Control Act of 1986 (IRCA). DHS now has "unfettered" authority to allow aliens to work in this country — or so the "rock solid legal argument" goes.
IRCA put in place the grand bargain that illegal aliens already in the country would get an amnesty and that, in exchange, the government would get serious on preventing people from entering the country illegally, including clamping down on employers who hire illegal aliens.
IRCA imposed criminal and civil penalties on employers who hire unauthorized aliens. That section is found at 8 U.S.C. § 1324a. A provision at the end of this section (1324a(h)(3)) defines the term unauthorized alien:
(3) Definition of unauthorized alien
As used in this section, the term "unauthorized alien" means, with respect to the employment of an alien at a particular time, that the alien is not at that time either (A) an alien lawfully admitted for permanent residence, or (B) authorized to be so employed by this chapter or by the attorney general.
Did you notice how this provision "explicitly" grants DHS the authority to allow anyone to work in United States?
Few people outside the Obama administration and New York Times editorial board do.
It looks like DHS sent a team of junior lawyers to scour the immigration system to find some justification for the new powers — and this was the very best they could come up with: "or by the attorney general" (now the secretary of Homeland Security, after the transfer of most immigration responsibilities out of the Department of Justice when DHS was created).
If you read the statute in context, you will find that IRCA contains seven specific provisions for (now) DHS to grant work authorization to aliens without work visas. In the absence of the phrase "or by the attorney general", such work permits would be useless because employers would be prohibited from hiring aliens possessing them. It is those specific provisions that "or by the attorney general" refers to; the phrase does not confer a general right to grant work permits to anyone the administration wants.
As Aesop wrote about this type of analysis in the fable of "The Wolf and the Lamb": "Any excuse will serve a tyrant."