On 27 January 2017, US President Donald Trump has issued an executive order entitled 'Protecting the Nation from Foreign Terrorist Entry into the United States'.
On 30 January 2017, the acting Attorney General at the time refused to defend it in court because it was unconstitutional or otherwise illegal: [news article].
On what basis(es) might this executive order be illegal?
A few possible reasons it could be illegal (on an issue spotting basis, not a careful analysis of each possible reason):
The EO is intended to discriminate on the basis of religion and in fact does so in violation of the 1st Amendment to the United States Constitution.
The EO is intended to unlawfully discriminate based upon race or ethnicity in violation of U.S. statutes or the 14th Amendment.
The EO was adopted without observing the notice and hearing requirements of the Administrative Procedure Act.
The EO took effect prematurely because it was not duly published in the Federal Register for the time period required by law, or was otherwise insufficiently promulgated.
The EO repeals other regulations currently in force in violation of the Administrative Procedures Act.
The EO improperly disturbs the vested, albeit limited, rights of existing visa holders (or visa applicants) without due process of law, and/or in violation of unenumerated 10th Amendment rights.
Refugee visa applicants under an already adopted regulation are entitled to an individualized consideration of their application as a matter of due process and the EO amounts to an ex post facto law as applied to people already in the application process under that regulation.
The EO improperly deprives people who purchased non-refundable plane tickets in reasonable reliance of their visa rights which were not revocable under any law in place at the time those tickets were purchased of a property right in those plane tickets without compensation or due process of law under the 5th Amendment.
The EO is void for vagueness as the ambiguities in its drafting makes it impossible to determine who is and is not affected by the EO and in what manner.
There is a conflict in the immigration statutes between statutory provisions and treaty obligations related to refugees, and the regulation implementing a particular section of the immigration law's grant of regulatory authority, and the refugee protections must prevail.
A person claiming to be a refugee has a due process right to individualized consideration of their circumstances, either by statute, by treaty, or under the constitution, which the EO does not respect.
The treatment of dual citizens under the EO either is contrary to statutory immigration law or to a treaty or to the U.S. Constitution and customary international law.
The EO violates one or more treaties between affected countries and the United States regarding immigration.
The scope and terms of the EO exceed the authority granted to the President by the section of the immigration statutes relied upon for authority to pass the EO.
The administration failed to articulate a rational basis for distinguishing between nations included in the ban and nations not included in support of the EO as required by the Administrative Procedures Act.
There is no rational basis for distinguishing between nations included in the ban and nations not included as required by the U.S. Constitution (a much weaker test since it allows for post hoc rationalizations).
Authority to implement the relevant section of the immigration law is vested in one or more officials at the United States Department of State, rather than the President directly, and none of those officials approved the EO.
The military interpreters and their families may have a contract with the U.S. government as part of their employment that gives them a right to a visa, in which case the EO would be a law impairing contracts in violation of the U.S. Constitution, as applied to them.
Some of the immigrants may need to accompany U.S. citizen children in order for the children to exercise the childrens' rights to enter the country and as applied the EO may violate the U.S. citizen childrens' rights.
The CBP may be incorrectly interpreting the EO (which even the administration is not clear about) and therefore violating the law as applied because the EO does not actually authorize their action or because the way that they interpreted the EO was an abuse of their discretion.
The manner in which the EO was implemented (e.g. long periods of time in handcuffs for civilians not accused of any crime and with no ability to foresee that they would be denied entry, separating young children from parents, etc.) may have been an unreasonable seizure as applied, even if detention per se was authorized. The 4th Amendment requires that searches and seizures be reasonable in the manner that they are carried out even if the search or seizure is itself authorized by law.
UPDATE February 2, 2017: A federal judge in California has found that at least some of these reasons have merit and has stayed the EO and set an expedited hearing and briefing schedule on whether the injunction against the EO should remain in force.
UPDATE AND EPILOGUE June 28, 2018: The final version of the ban was ultimately upheld by the U.S. Supreme Court in the case of Trump v. Hawaii on June 26, 2018 in a 5-4 decision. Litigation of the version of the ban in the OP was dismissed as moot when it was withdrawn and replaced with a similar but newer version of the EO. The four justices in the dissent rely on the first reason given, as did lower courts in the case and in parallel litigation. A core distinction between the majority and the dissent is the weight given by statements made by President Trump about the policy, and how much deference to give to the government's reasons for the policy offered up in court. Justice Neil Gorsuch, who cast a deciding vote in the case, took office April 10, 2017, after this answer and the first update to this answer were written.
Rather than focus on the particular Executive Order, I will consider the general grounds on which an XO may be unlawful.
Federal jurisdiction
First, it must deal with matters that are properly within the power of the Federal government. An XO that deals with matters that properly belong to individual states would be unlawful. This applies to all lawmaking powers of the US government, not just XOs.
Legal source of Power
In theory, the three arms of government have the following roles:
In practice, things aren't that clear cut:
Getting back to XOs, the President must have a legal basis for exercising this power. This can come from:
Content
The content of an XO must stay within the boundaries of the legal source of power, basically, it cannot go against any pre-existing law. If congress or the courts have decided that the law is X, an XO cannot say that it is Y. An XO can, however, change the law made by a previous XO by the same or another President, just as Congress can repeal or amend legislation and the courts can go against judgments that they previously made (at at the same or higher level of the court hierarchy).
