"And a stranger shalt thou not oppress; for ye know the heart of a stranger, seeing ye were strangers in the land of Egypt." Exodus 23:9
President Trump recently published an Executive Order limiting entrance into the United States by individuals from Iraq, Iran, Syria, Libya, Yemen, Sudan and Somalia, except under certain circumstances (e.g., diplomatic Visas) and suspending certain Refugee programs.
Is This Constitutional?
1. Does the President Have the Authority to Do This?
The President has power, under the "Take Care" Clause, to enforce the existing laws made by Congress.
This does not give the President power to create new law, but merely allows the President to detail how the law will be enforced under given circumstances. Pres. Carter did something similar to President Trump's order when he cancelled the visas of Iranian students during the Hostage Crisis in 1980 as did Pres. Obama when he slowed down the admission of certain Iraqi refugees in 2011 after some individuals admitted under that program were involved in terroristic activity.
2. Does this Comport with the Equal Protection Clause of the Fourteenth Amendment?
More of a question, but probably.
This is not a "Muslim Ban," as it temporarily halts "immigrant and nonimmigrant entry" from those countries, notwithstanding the religion of the person trying to gain entry.
Further, it is not even clear if the provisions of the First Amendment protecting 'free exercise" of religion would even apply to non-citizens seeking admission to the US.
Almost certainly, however, you could attack a "Muslim Ban" as "establishment" or a restraint on "free exercise" under the First Amendment as a limit on the federal government power rather than as asserting a citizen's right. However, this is not a "Muslim Ban" so that point is moot.
Possibly, as some people have attempted to do, you could argue that there is no nexus of operative facts linking immigrants and refugees from these countries to terrorism in the US.
For example, Tsarnaev Brothers who bombed the Boston Marathon were Chechens, Omar Mateen, who shot several people in an Orlando LQBTQ nightspot was the American-born son of Afghans and MAJ Nidal Hassan was the American-born son of Palestinian Immigrants (as well as being a Doctor, a Field Grade Officer, a former US Army Enlisted Soldier and, like Steve Bannon, a product of the Virginia Tech Corps of Cadets).
However, there have been incidents in the US involving Somali-American immigrants (the Mall of the Americas stabbing incident and the Ohio State stabbing incidents in 2016) and there were incidents involving Iraqis that lead to the 2011 slowdown ion processing Iraqis, ordered by President Obama in 2011.
A somewhat related argument is that other predominately Muslim countries (where there is a nexus to terrorist acts) are not banned, notably the Kingdom of Saudi Arabia ("KSA") (15 of the 19 hijackers were Saudi nationals) and Pakistan (where the ISI was a major early force in the formation of AQ).
However, the common thread between the nations on the list is that they are either hostile states (Iran, which has been in a state just less than war with the US for most of the period from 1979 and the present) or are failed or fragile states. In contrast, both the KSA and Pakistan are sovereign states that have cooperated in screening immigrants, at least since 9-11.
Neither a hostile state nor a failed state is likely to cooperate effectively in screening immigrants or refugees. As counterterrorism expert Seb Gorka has said these nations had been of concern to the Obama Administration, as well.
Arguably, Iraq is neither hostile nor failed. It is an existing state that is somewhat allied with the US like Afghanistan. However, like Syria, a considerable (but declining) portion of its territory is occupied by the Islamic State ("IS").
In fact, in 2015, when Fallujah and Ramadi fell to IS, the Government of Iraq held up refugees from entering Baghdad until they were screened. (Which may not have worked, as Baghdad has been hit by terrorist bombing several times since.)
3 . Does This Pass Muster Under the Due Process Clause?
The roll out of the program was a catastrophe, with people with valid visas being detained without proper process. This is why a judge from the EDNY enjoined enforcement. However, this kind of issue can be much more easily addressed than a claim that this beyond the President's authority or violated Equal Protection.
Does Doing This Make Any Practical Sense?
As the Hmong people can attest, the US has not always done overly well in keeping promises to local people who help us in our "Savage Wars of Peace." Anything that causes us to abandon people like Iraqi translators, or even tribesmen who supported us during the Anbar Awakening, is a blot on our national escutcheon and a potential Information Operations ("IO") windfall for IS and AQ.
Refugees from Syria (and IS occupied areas of Iraq) are a bit of a different case.
Although the vetting process for refugees can take up to two years, it is not the kind of screening that was done during the Cold War with defectors from the East ("Strategic Debriefing"). Given some incidents that have occurred in Europe, we should set up screening processes at least as rigorous as those used for defectors during the Cold War.
This is both like and dissimilar to the MS St. Louis/"Voyage of the Damned" incident in 1939.
Like the MS St. Louis incident, our barring refugees provides IS and AQ with a potential IO victory.
On the other hand, there is a palpable risk in admitting unvetted immigrants or refugees in this situation not present with refugees from the Nazis. So long as the ban is only temporary (and effective approaches to screening and vetting are developed), this is probably a sound practical course of action.
A More Controversial Issue
A more controversial issue that this does not address is why immigrants or refugees or the children of immigrants or refugees (like Hassan, the Tsarnaev Brothers or Omar Mateen) become so disaffected? While these are only a small minority of this entire population, any such event creates an IO opportunity for AQ and IS in what is probably an IO war where the "key terrain" is the "Arab (or better, "Muslim") Street."
Does the US no longer have a distinct or powerful identity that can bind immigrants or refugees?