This article will appear in the Summer 2016 issue of The American Prospect magazine. Subscribe here.

On December 7, in a seemingly throwaway line, Donald Trump said that if he were president he would move to bar all Muslims from entering the country until we "can figure out what the hell is going on." No more Muslim immigrants. No more Muslim visitors. In other comments, he has toyed with the idea of making all Muslims already in the country register with federal authorities.

These are policies so extreme that most commentators have dismissed them as simple Trumpian bluster, unconstitutional from the outset, designed to appeal to a bigoted base but unlikely ever to be legally enforced. After all, discriminating so overtly by religion, barring 1.6 billion people from entering the country, would be inconsistent with America's rejection of religious discrimination and place the United States firmly outside international law, putting it in conflict with the founding charter of the United Nations and the International Covenant on Civil and Political Rights. Since World War II, the world's open societies have prided themselves on increasingly inclusive immigration policies as well as firm protection of freedom of religion. Trump's ideas, by contrast, are far outside the mainstream, instead echoing those of neo-fascists in Europe.

Extreme though they are, Trump's proposals should not be treated as mere rhetoric. Using existing immigration statutes and legal precedents, President Trump would, in fact, have plenty of ways of making life miserable for Muslims currently living in or hoping to come to the United States.

Such an attempt would likely start, says Boston College law professor Daniel Kanstroom, who specializes in constitutional and immigration law, with an expansive interpretation of Section 212 of the Immigration and Nationality Act. The security-related provisions of Section 212 allow U.S. officials to bar any "alien whose entry or proposed activities in the United States the Secretary of State has reasonable ground to believe would have potentially serious adverse foreign policy consequences for the United States." People may also be denied entry if they are involved in "any activity a purpose of which is the opposition to, or the control or overthrow of, the Government of the United States by force, violence, or other unlawful means." Section 212 provided the basis on which U.S. officials blocked Communists from visiting or moving to the United States during the Cold War.

Bill Clark/CQ Roll Call via AP Images

"If Congress wanted to exclude all Muslims, constitutionally they can," says Mae Ngai, a Columbia University historian of U.S. immigration policy who has written extensively on the Chinese Exclusion Act. "Congress can exclude anyone it wants to. Nobody has a right to enter the country."

Were a Trump administration to argue that America was essentially at war with large parts of the Muslim world and that Islam per se was an un-American, totalitarian ideology-a position already taken in public speeches by a number of state legislators in Oklahoma, Texas, and other states-it could then, conceivably, craft a legal argument that any Muslim was, by definition, a potential foreign policy threat or national security risk. It could empower consulate officers and officials at ports of entry to try to root out suspected Muslims and bar them from entry; and it could also then use national-security arguments to require all noncitizen Muslims in the United States to register with federal authorities, with deportation as the penalty for noncompliance.

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Would a Trumpian ban on Muslim immigrants and visitors, as well as measures directed at Muslim noncitizens in the United States, be challenged in court? Of course. In the interim, however, while the challenges were working their way up to the Supreme Court, it's conceivable that the lower courts would let such policies stand, says Drexel University constitutional and human rights law expert Anil Kalhan.

Carlton Larson, a legal historian at the University of California Davis School of Law, doesn't think the courts would accept measures directed at Muslims across the board. But he believes they might agree that adherence to particular strains of politicized Islam was ideologically incompatible with entry into, or continued residence in, the United States. Instead of calling for a ban on all Muslims, Trump might therefore focus on adherents of what he and other Republicans refer to as "radical Islam." While such a ban would not categorically exclude all Muslims, the government would then have grounds for inquiring into the beliefs of all Muslims-prospective immigrants and visitors as well as legal residents-to determine who among them held radical beliefs dangerous to national security.

There are, of course, precedents. It was on national-security grounds that Japanese Americans were interned following Franklin Roosevelt's signing of Executive Order 9066 on February 19, 1942. Limited numbers of German Americans were also temporarily detained during World War II. In both of these instances, citizens and noncitizens alike were targeted. The Supreme Court upheld the legality of Japanese internment as a national-security imperative in 1944, in a 6–3 decision in Korematsu v. United States, a ruling that has continued to be cited favorably by the Court. And in Narenji v. Civiletti, the Court ruled that the expulsion of Iranian students during the Iranian hostage crisis under President Jimmy Carter didn't violate the Equal Protection Clause of the Constitution.

The Supreme Court's decision in Korematsu introduced the legal standard of "strict scrutiny" for any race-based policy adopted by the government.

The Court applies the same standard to governmental measures involving religion. Under strict scrutiny, a policy based on any "suspect classification" must meet several criteria. Besides serving a compelling public purpose-which can be used by the government to argue that a policy doesn't violate the Equal Protection Clause-such a measure cannot be overbroad. Were a Trumpian administration to try to ban all Muslims from entering the country, on the grounds that the government had a compelling interest to protect the country from Islamist terrorists, lawsuits challenging the policy would argue the categories used were too vague. "Narrow tailoring is the magic phrase," says Yale law professor Ian Ayres. Courts have held, especially in recent cases involving affirmative action, that the policies must be narrowly tailored to further the government's compelling interest without ensnaring people unnecessarily. Ayres doesn't see how a blanket ban on all Muslims would ultimately withstand this test. But, like Larson, he does think that a ban could exclude followers of specific ayatollahs, and possibly members of particularly extreme sects.

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Post–9/11, the Bush administration developed a slew of programs, under an umbrella FBI initiative known as Pentagon/Twin Towers Bombing Investigation (PENTTBOM), which was designed to register, and monitor, groups of people who came to the United States from countries that were Muslim-majority and considered to be incubators of Islamist terrorism and extremist religious ideology. Among these programs was the National Security Entry-Exit Registration System (NSEERS), which required individuals from 25 countries to be registered, fingerprinted, and photographed. Ultimately NSEERS affected more than 80,000 people. The Bush administration presented NSEERS and related programs as vital to national security in an era when al-Qaeda was planning more attacks, and the program was deemed constitutional in a number of cases, in which evidence was presented in secret, closed sessions.

In 2003, after spending more than 18 months reviewing the publicly available evidence, researchers from the Migration Policy Institute concluded that the special registration requirements and tightening up of immigration enforcement-including roundups of Muslims and Arabs, 13,000 of whom were placed in deportation proceedings-had "failed to locate terrorists, and damaged one of our great potential assets in the war on terrorism: the communities of Arab- and Muslim-Americans." Nonetheless, the programs stayed in place, most of them until 2011, some still ongoing.

Dorothea Lange/National Archives/Public Domain The Supreme Court upheld the legality of Japanese internment as a national-security imperative in 1944, in a 6–3 decision in Korematsu v. United States, a ruling that has continued to be cited favorably by the Court. Here, members of the Mochida family awaiting evacuation bus in Hayward, California.

"We now have a broad array of measures that have targeted one particular slice of the noncitizen community: Arabs and Muslims," explains Susan Akram, director of the human-rights law clinic at Boston University. "There's a great deal of latitude in singling out Arab and Muslim communities in a way that would not pass muster for U.S. citizens."

Muslim citizens seeking to bring family members to the United States under the family reunification provisions of current immigration laws might be in a stronger legal position than prospective immigrants themselves to contest a ban on Muslim migration into the country. But then again, maybe not. In 2008, the Bush administration introduced the Controlled Application Review and Resolution Program (CARRP), which allowed federal agencies to go slow on approving visas, refugee or asylum status, work eligibility, and adjustment-of-status requests for family members from certain Muslim-majority countries. CARRP was introduced, in secret, with no congressional approval or oversight, by the Citizenship and Immigration Services. Since 2013, two suits have been filed alleging that the go-slow procedures amount to a decision to deny entry to people based on their geographic origins. One suit was withdrawn after the plaintiffs had their application processed; no ruling has yet been handed down in the other case. A freedom-of-information request filed by the American Civil Liberties Union of Southern California turned up evidence that between 2008 and 2012, CARRP affected more than 19,000 people globally. More recently, a Las Vegas–based journalist's FOIA request found that the number had reached nearly 42,000.

"The CARRP program directs agency officers to delay and ultimately deny the immigration benefits applications of applicants it has blacklisted, all without even telling these individuals that they were labeled threats to our nation, let alone giving them an opportunity to respond to the allegations," wrote Jennie Pasquarella, director of immigrant rights at the ACLU of Southern California, in the 2013 report "Muslims Need Not Apply." Applicants could be listed as "national security concerns" simply for attending a mosque the FBI had under surveillance or for traveling through an area of known terrorist activity.

While CARRP has been implemented largely in secret, UC Irvine School of Law professor Sameer Ashar believes a Trump administration could publicize its provisions and use it as a political tool to further limit Muslim entry. Pasquarella, too, believes it could be used to dramatically curtail the number of Muslim refugees entering the United States from Syria, Afghanistan, Iraq, and other war-torn regions of the Muslim world, as well as family members seeking to join relatives in the U.S. Trump could, she says, explicitly "tether it to religious criteria."

"AS A PRACTICAL MATTER, it's ridiculous; it's unenforceable," says Kanstroom about a ban on Muslims. "But let's imagine they can come up with a machine to identify Muslims.

The question is, is it legal? The Chinese Exclusion Act is a good starting point.

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Passed in 1882, the act excluded all Chinese laborers, though it allowed Chinese merchants, students, and ministers to enter the country as long as they carried with them what was known as a Section 6 certificate. Seven years later, a case came to the Supreme Court involving a man who had lived lawfully in San Francisco for 12 years and then was barred from re-entering the country after he returned to China to visit his family. The Court ruled that under the Plenary Power Doctrine, noncitizens seeking entry into the United States had no constitutional claims.

In 2017, the Plenary Power Doctrine would form a core part of the Trumpian legal argument that America could bar entry to people using religion as a criterion. Under Plenary Powers, Muslim nonpermanent residents on work visas, student visas, and so on could conceivably find themselves denied permission to re-enter America should they temporarily leave, says Ashar. Theoretically, so too could some citizens. In recent years, a handful of U.S. citizens who traveled to Yemen, presumably to liaise with al-Qaeda, have had their passports either seized or revoked by embassy officials in Yemen, thus effectively preventing their re-entry into the United States.

But all of this leaves the huge question of how one determines who is, in fact, a Muslim. Is "Muslim-ness" about beliefs in God or simply a set of cultural values? Is it about rituals followed or the name on one's birth certificate?

Putting government in the business of religion-testing would almost certainly involve consular officers asking visa applicants questions about their beliefs, the holidays they observed, their dietary habits, their parents' religious affiliations, and other similar questions.

"Can you force someone to eat or not eat something?" asks the writer and creative director of Affinis Labs, Wajahat Ali, who has written a number of reports on the growing dangers of Islamophobia in America. "Do you dangle a pork sandwich in front of them? Do you check backpacks for Halal items?" Is the TSA going to have a circumcision-inspection squad? Conceivably, says Ali, if the rationale is "anyone who is a Muslim is a potential suspect, and we'd rather be safe than sorry."

Demagogues like Trump who target outside groups usually have a core set of hatreds. What shines through Trump's public statements-from his fetishizing the idea of torturing terror suspects to his advocacy of dipping bullets in pig's blood and using them to summarily execute Muslim terrorists, from his support for collective punishment against Muslims to his statements that Muslims as a group hate America-is his animus against Muslims. The more he expresses his dislike of Muslims, the more he empowers every tin-pot race- and religion-baiter to come out and not just publicly espouse intolerance but act on it. Trump is creating a culture that invites violence against Muslims, and if the history of official targeting of minorities is any indication of what may happen, his proposals need to be taken extremely seriously. For if elected, he will need more red meat to throw to his energized mob to keep them on board as he builds his power base.

AP Photo/Brynn Anderson

This kind of popular agitation makes court rulings unpredictable and less dependent on past precedent. The historian of anti-homosexuality law William Eskridge has documented the moral panics that fueled campaigns against free-love advocates, contraception, and feminism in the 19th century, as well as anti-homosexuality laws, prosecutions, and employment witch hunts in the decades after World War II. How did these laws withstand 14th Amendment challenges? he asks in his 2008 book Dishonorable Passions. Because, he answers, "legislators and judges came to understand the Fourteenth Amendment pragmatically, in light of public opinion."

If Trump-with the full powers of the presidency at his disposal-convinces enough people that Muslims as a group constitute a major safety risk, a threat to public order, and a potential Fifth Column, Akram believes the courts wouldn't necessarily act as a buffer.

Could a Trumpian Congress-or a non-Trumpian Congress intimidated into acquiescence by a populist president's willingness to tap into the culture of fear and to exploit public reactions to ongoing acts of Islamic terror on U.S. soil-define Islam as a national-security threat? Could distributing the Quran be considered proselytizing for an un-American political system? Such proposals have already been mooted by groups such as the Center for Security Policy, as well as legislators in Oklahoma such as John Bennett and Sally Kern.

About two-thirds of foreign-born Muslims in the country are naturalized citizens, and they would be protected by the First Amendment's provision for "free exercise" of religion. But that protection has not always been rigorously observed. In the 19th century, Congress crafted several anti-Mormon laws-officially defined as being anti-polygamy rather than anti-Mormon-that had the effect of drastically curtailing the franchise and other rights of Mormons.

That the courts upheld such laws was at least in part because large sections of the population were deeply hostile to Mormonism. "The normative landscape can shift dramatically around a set of legal opinions based on how public opinion is mobilized," argues Kalhan. "When highly politicized questions go to court, all bets are off."

A blanket denial of entry to Muslims would likely have ricochet effects in other areas. After all, if the Justice Department is arguing in court that Muslims as a group are a national security threat, surely they shouldn't work for the government or in the military or hold sensitive scientific or industrial positions. And if Muslims pose a danger to the nation's security, surely communities should have the right to exemptions from Fair Housing Act requirements so as to be able to bar Muslims from living in their midst.

If Trump's dislike of Muslims is as intense as his public comments suggest it to be, it isn't beyond the bounds of the possible that, step by step, Muslims' status in the country, as well as their daily security, would be undermined during the first years of his presidency.

Could we end up with attempts to ferret out closeted Muslims, as vice squads did homosexuals in decades past, or as the Spanish Inquisition's investigators did against Conversos (Jews who had allegedly converted but who in secret still adhered to Jewish rituals)? Surely not in 21st-century America. Then again, that's what rational, modern-minded, enlightened Germans would have thought, in the twilight days of Weimar, about the possibility of medieval religious codes being reintroduced against Jews. Things can get ugly, Larson concludes, "if you have people mobilized into a raging froth of hate."