Leading Republican presidential candidate Donald Trump earlier this week proposed a ban on all Muslims entering the U.S. Trump, whose proposal was overwhelmingly condemned both at home and abroad, comes in the wake of the recent terror attack in in San Bernardino, California.

Wendy Parmet, the George J. and Kathleen Waters Matthews Distinguished University Professor of Law, says such a ban would fly in the face of U.S. constitutional values, morality, and human rights.

Is Donald Trump’s proposed ban of all Muslims entering the United States, whether they are migrants, refugees, or tourists, illegal? If so, what part of the Constitution disallows it?

Any attempt to ban Muslim citizens from entering the U.S. would clearly violate the Fifth Amendment to the Constitution. Trump’s proposal to bar non-citizen Muslim migrants, refugees, or tourists likewise offends many well-established constitutional values, including respect for the free exercise of religion and equal protection under the law. It also flies in the face of non-discrimination principles recognized in international human rights law. However, with respect to non-citizens outside of the country, the proposal is probably not unconstitutional under current Supreme Court doctrine as the federal government has almost unchecked authority in deciding which non-citizens may or may not enter the country.

Theoretically a court could halt the ban. But given the significant deference courts grant to the federal government in immigration cases, especially when brought by non-citizens who are outside of the U.S., it is far from certain that a court would grant such relief. Congress could also halt such a ban by passing a law forbidding the president to discriminate on the basis of religion. Of course, a president intent on such a ban might veto any such legislation.

Trump stated that he is no different from President Franklin Delano Roosevelt, referring to the internment of more than 100,000 people in U.S. government camps during World War II. How are these two instances similar, if at all, and how are they different?

The internment of Japanese Americans during World War II is one of this nation’s greatest injustices. In 1988, President Ronald Reagan signed a law apologizing to those who were interned. It is therefore shocking that Trump would rely upon this wrong to justify his own proposal.

There are, however, many similarities between the two situations. Perhaps the most salient is that both the internment and Trump’s proposal demonstrate the potency of prejudice in times of fear. During World War II, as today, many people were quick to blame Japanese Americans without any evidence that they posed any threat whatsoever. This same rush to scapegoat, separate, and keep out a minority population seems to be at work today.

As for differences between Trump’s proposal and the internment of Japanese Americans, two stand out. First, in some sense the internment was more draconian. Both citizens and non-citizens were confined in internment camps. Trump, in contrast, is simply calling for a ban on non-citizens entering the country. On the other hand, the internment occurred during a World War. Although terrorism poses a real threat, we are not in a real world war, and the magnitude of the risk does not compare to that in World War II. In that sense, the fear and scapegoating that is occurring today is even less explicable than that in World War II.

What difficulties arise from attempting to make religion part of the screening process for those trying to enter the country?

It’s hard to imagine how Trump’s proposal could be implemented without further subverting deeply held constitutional values. In order to keep Muslims out, all immigrants and travelers would have to be asked about their religion either when they seek a visa or upon entry to the U.S. Many people, Muslim and non-Muslim, would find this deeply offensive.

Moreover, to keep Muslims out, immigration authorities would have to be able to check if people gave truthful answers to the question “Are you a Muslim?”. Terrorists, I assume, would not always give truthful answers. How would authorities decide if the answers given were truthful?  Would they decide that someone who claims not to be a Muslim was one because her father was one, or because she attended a mosque as a child? Would we divert intelligence resources into finding out who is a practicing Muslim? Even if that were practical and sensible—and it is neither—the idea that the federal government would develop religious tests and label people accordingly is deeply offensive to our heritage of religious tolerance.