The Fifth Amendment of the US Constitution provides that the Federal government may not deny any person due process of law. The Supreme Court holds that "due process of law" includes the right to "equal protection" under the law, as described under the Fourteenth Amendment.
In a sense, all laws must treat different classes or categories of persons differently. For example, those who commit crimes go to jail, and in that way are treated differently from those who do not commit crimes. Where a law runs afoul of the Fifth Amendment or Fourteenth Amendment, however, is when the different treatment in question is unrelated to a legitimate government purpose.
There are multiple tests to determine whether a law is related to a legitimate government purpose, based on whether the classification of persons treated differently falls into a "suspect" category. For example, differentiating partnerships from corporations is not inherently "suspect", so a law that treats the two businesses unequally will NOT violated the Fifth or Fourteenth Amendment IF the law is "rationally related" to any "legitimate" government interest.
On the other hand, if the law treats people differently based on a "suspect" classification, a higher level of scrutiny applies. Laws that treat blacks differently from whites are inherently "suspect" and must undergo strictest level of scrutiny in order to survive a Fifth Amendment or Fourteenth Amendment challenge. Such a law would have to be "narrowly tailored" to satisfy a "compelling" government interest, and must be the "least restrictive" means of doing so. Few laws would survive such a stringent standard of review. It is this same strict level of scrutiny that applies to all laws that treat people differently based on their race, religion, national origin, or alienage. With a caveat.
This past week, Donald Trust issued an executive order that halts the issuance of either immigrant or non-immigrant visas to nationals of 7 specified countries. In so doing, the order provides unequal treatment of persons based on alienage or national origin - giving rise to obvious Fifth and Fourteenth Amendment issues.
Surprisingly, the requisite level of judicial scrutiny to determine the constitutionality of Trump's order is NOT whether his order is "narrowly tailored to satisfy a compelling government interest in the least restrictive means possible." Instead, the test is the least stringent standard of review - to wit, it is whether Trump's order is "rationally related" to a legitimate government interest. The reason for this relaxed level of scrutiny is because the Constitution expressly provides Congress with the authority to regulate immigration - and Trump's order is very carefully tailored around the Immigration and Nationality Act. The team of lawyers who drafted this order are clearly well versed in Constitutional law, and if you were only looking at Trump's executive order itself, there might be strong arguments that support the constitutionality of Trump's order from the perspective of the Fifth and Fourteenth Amendments. We will turn back to this in a moment.
But frst, let's turn our attention to whether the order violates the First Amendment, which proscribes Congress from restricting the free exercise of religion. The word "Muslim" does not appear anywhere in the text of Trump's order, but the fact will not be lost on any court that the impact of the order focuses squarely onto Muslim majority countries, and will disproportionately impact Muslim travelers. In so doing, a law preventing people from entering the USA based on their religion would certainly restrict their right to freely exercise that religion. On it's face Trump's order may survive a First Amendment challenge, whereas in application, the question becomes less certain. Trump has vocally touted his plan to bar Muslims, specifically, from entering the country - so the "legislative history" behind his executive order is damning to say the least, and most clearly was designed to target individuals based on their religion and repress their religious liberty in direct contravention of the most basic American freedom. But in ascertaining the constitutionality of his executive order, will a court take Trump's campaign language into consideration? Uncertain, but I would guess that most courts will be inclined to review all the facts and circumstances surrounding this executive order, how it is implemented, against whom it is implemented, and why it was enacted (whether or not explicit in the language of the order).
In short, Trump's order is very carefully drafted, and does not on it's face necessarily violate the black letter of the US Constitution. Arguably, the order violates the spirit of the US Constitution (and offends any descent sense of human dignity), but that does not automatically render the order unconstitutional.
Where Trump's legal team went wrong was by looking within the four corners of the proposed executive order, without contemplating the impact that the order might have on already-existing immigration law. Had they done so, they would have quickly found that Trump's executive order patently violates Section 202(a) of the Immigration and Nationality Act of 1965 (commonly known as the Hart-Celler Act), which expressly provides (in part) that no person shall receive any preference or priority or be discriminated against in the issuance of an immigrant visa because of his race, sex, nationality, place of birth or place of residence (subject to certain quotas and preferences for family members and other general requirements not specific to a person's race, gender, nationality, etc.) In plain English, the President has no power to bar people from entry into the country based solely on their national origin or place of residence. The visa waiver provisions that Trump's order purports to rely upon do not in any way modify or supersede Section 202(a) of the Immigration and Nationality Act. Quite simply, Trump's legal team may have done an admirable job researching the law as it applies to the proposed executive order itself, but entirely failed to consider the law OUTSIDE of the four corners of the executive order. That was a fatal mistake, from a Constitutional law perspective.
That Trump's order violates Hart-Celler act seems entirely free from any doubt, and that brings us right back to the Due Process clause of the US Constitution. "Due Process of Law" means that the process in question must be legal, and based on Section 202(a) of the Hart-Celler Act, Trump's order is most clearly NOT legal. Any attempt to enforce Trump's order is, for that simple reason, a per se violation of the Due Process Clause of the US Constitution.
To sum up, we have to evaluate the Constitutionality of Trump's executive order by considering BOTH the order itself, the facts surrounding it's adoption, and most importantly, the fact that it fails to comply with the express provisions of the 1965 Hart-Celler Act. In the days and weeks to come, I suspect we will hear more about the 1965 Immigration and Nationalization Act, since it presents the clearest "open and shut" argument against Donald Trump's order.