Newt Gingrich effectively called for the deportation of all practicing Muslims from the United States Thursday night. His suggestion, which would almost certainly be unconstitutional, represents perhaps the most sweeping call for a religious test proposed by a mainstream political figure over the last few years, which have seen a strong backlash against Islam.

“Let me be as blunt and direct as I can be. Western civilization is in a war. We should frankly test every person here who is of a Muslim background, and if they believe in sharia, they should be deported,” Gingrich said on Fox News. “Sharia is incompatible with Western civilization. Modern Muslims who have given up Sharia—glad to have them as citizens. Perfectly happy to have them next door.”

Gingrich continued by saying that anyone who visited a website associated with ISIS or Al-Qaeda should be prosecuted as a felon.

Gingrich has a tendency to speak off the cuff, often saying outrageous things, so it’s hard to tell how considered the comment was. But it represents a grave misunderstanding of both the First Amendment and “sharia,” an oft-blurred term.

On the first count, it’s hard to imagine Gingrich’s suggestion passing anything resembling constitutional muster. Presumptive Republican nominee Donald Trump has advocated for a ban on Muslims entering the United States, and legal opinions diverge on that; some scholars believe that such a prohibition might be upheld in courts, based on immigration laws. What Gingrich is suggesting is radically different. He would not only change who the United States allows in; he would apparently seek to deport those already resident, including perhaps American citizens, “of a Muslim background,” and he would do so on the basis of a religious test, despite the First Amendment’s guarantee of freedom to practice religion. (Would Anglo-Saxon or African American converts to Islam be spared?)

Such an approach would also be difficult to enforce. What would prevent a believer from simply lying? As many an anti-Islam campaigner has pointed out, Shia Islam grants believers a dispensation to conceal their faith in the face of persecution. In some cases—the Spanish Inquisition comes to mind—elaborate tests have been devised to sniff our secret Muslims and Jews, but those tests were both unscientific and obviously brutal; and even so small colonies of Muslims persisted in Spain, perhaps as late as the 1980s.

Are there any precedents for this in American history? The common parallel cited is the internment of Japanese Americans at the start of World War II, an incident that has been widely regarded as black mark on the nation’s history, if one that Trump has cautiously defended. But even that ban was based on ethnicity tied to a nation with which the U.S. was at war, not a religious identity spanning the globe.

Gingrich’s suggestion also represents a common if serious misunderstanding of sharia. “Sharia” is often used as a shorthand for radical Islamism, but Gingrich shows the limitation of that approach. To suggest that Muslims are acceptable as long as they don’t believe in sharia is a little like saying Christians are OK as long as they don’t believe the Gospel. In other words, any practicing Muslim believes in sharia. It’s simply a code of behavior and law in the religion, just as in any other religion. And just as there are Christians whose religious beliefs run the gamut from casual belief and church attendance on Christmas and Easter to those who commit violence and murder in the name of their religion, there is a range of Muslim beliefs that fit under the rubric of sharia. Might a reasonable Christian disavow someone like Eric Rudolph, insisting he represented a perversion or misunderstanding of what Jesus taught? Of course. But then most American Muslims would (and do) say the same of Islamist terrorists.

Gingrich’s latest comments are related to things he’s said before. In 2006, for example—at a dinner celebrating the First Amendment, no less!—the former House speaker argued that freedom of speech needed to be reconsidered in light of the threat of terrorism. Gingrich spoke out against the so-called Ground Zero Mosque, a proposed Islamic center in lower Manhattan, and he has advocated for banning the use of sharia in American courts. My colleague Conor Friedersdorf in 2012 rounded up some of Gingrich’s comments about Islam and Muslims, arguing that if they had been made about any other religious group, Gingrich would have been drummed out of polite circles.

But the idea of deporting any believing Muslims, including apparently U.S. citizens, is a major leap. It puts him well beyond even Trump, who previously said he might support a registry for Muslims but was unable to explain how such a registry would differ from Nazi Germany’s policies toward Jews. Gingrich has been said to be a top contender to be Trump’s running mate, but on Thursday it began to appear that Trump would instead choose Indiana Governor Mike Pence. In the wake of the Nice attacks, Trump postponed his announcement, which had been planned for Friday morning. Should he decide the attacks demand a more bellicose, anti-Islam vice-presidential candidate, Gingrich has made the case for himself.