Donald Trump — who has spent his months atop the Republican polls slowly replacing the anti-immigrant rhetoric in his campaign with anti-Muslim rhetoric — is now officially calling to ban all Muslims from entering the United States.

His campaign has clarified that this would apply to "everybody," mentioning in particular Muslims entering the US as immigrants or tourists. Presumably, "everyone" also includes people entering on temporary work or student visas. His campaign initially claimed Muslim American citizens who are currently traveling abroad would be prevented from reentering the US (something that has already happened in isolated cases, though not deliberately, to citizens placed on the no-fly list), but in a talk-show appearance Tuesday Trump said Muslim citizens would be allowed to return.

Trump wants the ban to hold "until our country's representatives can figure out what is going on." Here's the full statement:

Donald J. Trump Statement on Preventing Muslim Immigration (New York, NY) December 7th, 2015, -- Donald J. Trump is calling for a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what is going on. According to Pew Research, among others, there is great hatred towards Americans by large segments of the Muslim population. Most recently, a poll from the Center for Security Policy released data showing "25% of those polled agreed that violence against Americans here in the United States is justified as a part of the global jihad" and 51% of those polled, "agreed that Muslims in America should have the choice of being governed according to Shariah." Shariah authorizes such atrocities as murder against non-believers who won't convert, beheadings and more unthinkable acts that pose great harm to Americans, especially women. Mr. Trump stated, "Without looking at the various polling data, it is obvious to anybody the hatred is beyond comprehension. Where this hatred comes from and why we will have to determine. Until we are able to determine and understand this problem and the dangerous threat it poses, our country cannot be the victims of horrendous attacks by people that believe only in Jihad, and have no sense of reason or respect for human life. If I win the election for President, we are going to Make America Great Again." - Donald J. Trump

Trump's announcement comes as the FBI has uncovered evidence that the two Muslim American attackers who killed 14 people in San Bernardino, California, last week were both "radicalized" and "had been for some time." One of those attackers, Syed Farook, was born in America. The other, his wife Tashfeen Malik, was born abroad but came to the US to marry Farook.

Is it constitutional to prevent all Muslims from entering the US?

It certainly sounds unconstitutional — the Constitution guarantees the free exercise of religion for all Americans. But when it comes to approving visas to come to the US, the government has a lot of leeway: It doesn't have to tell someone why his or her visa (or application to immigrate permanently) was denied. So in practice, the US could probably quietly deny Muslims' visa requests for a while. But announcing it in advance would probably be unwise, and the policy might get challenged in court.

Furthermore, tourists visiting from countries participating in the visa waiver program, which allows short visits from certain countries without a visa at all, don't have to apply for visas — so the government wouldn't have the opportunity to quietly bar them from entering. (Ironically, if Congress overhauls the visa waiver program — something both it and the White House think needs to happen in the wake of the Paris attacks last month — this would get easier.)

Trump's been slowly escalating his Islamophobic rhetoric for several weeks

Donald Trump climbed to the top of the GOP presidential polls in part on the strength of his attacks on Mexican and Latino immigrants. But as the summer has turned to fall, Trump — partly in response to news events, and partly to maintain his relevance with a political establishment (and political press) that's been frankly desperate to stop paying attention to him — has turned his focus to Muslims and Muslim Americans.

Trump was weeks ahead of his rivals in calling for the US to stop accepting Syrian refugees — as early as October, he was saying in stump speeches that refugees could be a "Trojan horse" for ISIS and that they should be sent back to Syria to fight there. (Other Republicans adopted variations on this position after the Paris attacks; at least one of the attackers was able to reenter Europe on a fake Syrian passport after leaving.)

In November, Trump began to call for particular mosques to be shut down. He flirted with the idea of requiring all Muslim Americans to register in a database. And just last week, in a speech to the Republican Jewish Coalition, he offset his relatively blatant anti-Semitism with an insinuation that Obama himself is Muslim: "Our president doesn’t want to use the term [...] ’radical Islamic terrorism.' There is something wrong with him that we don’t know about."

Trump is tapping into a wave of genuine Islamophobia in the US

Trump justified his call for a Muslim ban by citing a poll that purports to show 25 percent of Muslims believe violence against America is justified. That poll is totally bogus; it comes from the organization of Frank Gaffney, a man who once said that the US missile defense logo is evidence of President Obama's "submission to shariah," and it's riddled with methodological flaws.

But the real reason for Trump's statement — and the reason it resonates so much — is because of a different set of poll results: those showing rising Islamophobia in America.

Americans are more afraid of terrorism than they used to be: Even before the Paris attacks, they were more fearful of becoming victims of terrorism than they had been at any point since the immediate aftermath of 9/11. This is partly because Americans are just more worried, period; they're also more afraid of crime, more worried about race relations, and more concerned about illegal immigration.

They are also less tolerant of Muslims: A survey from the Public Religion Research Institute in September 2015 found that a majority — 56 percent — of Americans think Islam is "at odds with American values and way of life," up from 47 percent in 2011.

This poll was not an outlier, as my colleague Max Fisher has documented:

A February poll showed that 54 percent of Republican respondents believe that Obama "deep down" is best described as Muslim. By September, an Iowa poll found that only 49 percent of Republicans there believed that Islam should be legal, with 30 percent saying it should be illegal and 21 percent "unsure." Among Trump supporters in Iowa, hostility toward Muslims was higher but not that much higher: 36 percent said Islam should be outlawed… Fifty-seven percent of Americans, and 83 percent of Republicans, say that Muslims should be barred from the presidency.

This isn't just about polls, either. Last week, Max documented several examples of Islamophobic violence and threats of violence over the past several months. Here's his conclusion:

The United States does not officially track the number of citizens who are Muslim, but it's likely a few million. A 2010 Pew survey estimated the Muslim American population at 2.6 million and predicted that by 2030 it would rise to 6.2 million, or about 1.7 percent of the population. These Americans increasingly live in a climate where they face not just hateful and discriminatory rhetoric but also violence and the threat of violence. The threat of violence often has the same theatrical point as violence itself: to terrify and intimidate, to inflict psychological suffering on the targeted group in the form of fear and alienation, to force that targeted group to live a little bit less in the open and more in the shadows. Obviously, actual attacks on Muslims are worse than implicit or explicit threats — and, make no mistake, when militias stand outside an Islamic center, even if they have no intention of using violence, they are conveying a threat — but they serve the same goal of inflicting suffering on Muslims meant to drive them into the shadows or out of public society altogether.

The day after Fisher wrote this piece, the San Bernardino attacks happened — and law enforcement subsequently discovered they'd been carried out by a pair of radicalized Muslim Americans. And from what we've seen after the shootings, it certainly doesn't seem like Islamophobia is going to lessen, to say the least. As Jennifer Williams catalogued, it didn't take long for coverage of the San Bernardino attacks to turn Islamophobic. President Obama was worried enough about rising Islamophobia that he felt the need to give a primetime Oval Office address Sunday night partly warning Americans not to do exactly what Trump is encouraging them to do. (For what it's worth, the White House wasted no time in condemning Trump's Muslim ban proposal: Deputy National Security Adviser Ben Rhodes said it was "totally contrary to our values as Americans.")

But the bottom line is this: Donald Trump would not be attracting a plurality of support among primary voters as a presidential candidate if he weren't saying things many Americans believe. It might be unconstitutional to turn those beliefs into government policy, but that doesn't make them any less dangerous to American Muslims who could be the victims of emboldened Islamophobic violence.

Watch: President Obama's speech