The proposal, which was quickly denounced by other candidates from both parties, marks the latest escalation of the virulent anti-Muslim and anti-immigrant rhetoric that has fueled his unlikely candidacy. It also came less than 24 hours after President Obama urged tolerance in an Oval Office address, saying the fight against terrorists should not “be defined as a war between America and Islam.”

Supporters of Donald Trump wave to the press after being prodded by Trump during a rally Monday night aboard the aircraft carrier USS Yorktown in Mt. Pleasant, S.C. The crowd cheered Trump's call for a ban on Muslims entering the U.S. The Associated Press

CANDIDATES RESPOND

“Donald Trump is unhinged. His `policy’ proposals are not serious.”
Jeb Bush, former Florida governor, via Twitter

“Well, that is not my policy. I’ve introduced legislation in the Senate that would put in place a three-year moratorium on refugees coming from countries where ISIS or al-Qaida control a substantial amount of territory. And the reason is that’s where the threat is coming from.”
Ted Cruz, Texas senator

“There are folks in this race who don’t care about what the law says because they’re used to being able to just fire people indiscriminately on television. So, they don’t have to worry about what laws say or not say.”
Chris Christie, New Jersey governor

“Trump’s overreaction is as dangerous as President Obama’s under-reaction.”
Carly Fiorina, former technology executive

“Everyone visiting our country should register and be monitored during their stay as is done in many countries. I do not and would not advocate being selective on one’s religion.”
Ben Carson, retired neurosurgeon

“This is reprehensible, prejudiced and divisive. (at)realDonaldTrump, you don’t get it. This makes us less safe.”
Hillary Clinton, former secretary of state, via Twitter.

“Demagogues throughout our history have attempted to divide us based on race, gender, sexual orientation or country of origin. Now, Trump and others want us to hate all Muslims. The United States is a great nation when we stand together. We are a weak nation when we allow racism and xenophobia to divide us.”
Bernie Sanders, Vermont senator

But at a rally Monday night in South Carolina, Trump received a boisterous standing ovation as he shared the idea, telling the crowd that a ban is “common sense” and that his Muslim friends agree with him.

“We have no choice,” he said to cheers. “We have no choice. We have no choice.”

Party leaders have worried for months that Trump’s divisive rhetoric could poison their party, even as it has allowed him to maintain a commanding lead of the divided field since summer. Trump has broadly described illegal immigrants as violent criminals, called for the immediate deportation of the 11 million undocumented immigrants living in the United States and, at a recent rally, seemed to condone an assault on a Black Lives Matter protester.

Many legal scholars said such a plan would violate both U.S. and international law and would never be allowed by the courts, while leaders of Islamic groups called the idea outrageous.

“One has to wonder what Donald Trump will say next as he ramps up his anti-Muslim bigotry,” said Ibrahim Hooper, national communications director at the Council on American-Islamic Relations. “Where is there left for him to go? Are we talking internment camps? Are we talking the final solution to the Muslim question? I feel like I’m back in the 1930s.”

Trump’s proposal came in a statement issued on Monday 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.”

He said in the statement that “it is obvious to anybody the hatred is beyond comprehension.”

“Where this hatred comes from and why we will have to determine,” he continued. “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.”

Like most of the ideas Trump has floated, the proposal is both far-reaching and vague, raising numerous questions that his aides declined to answer Monday: Which Muslims would be included in the ban? How would they be identified? Would the U.S. bar American-born citizens who practice Islam and are returning from an overseas trip? What about holders of green cards visiting family overseas, or wealthy Middle Eastern businessmen journeying to the United States to finalize a deal?

Trump’s campaign manager, Corey Lewandowski, told the Associated Press that the ban would apply to “everybody” but did not elaborate. Later, Trump said in an interview on Fox News that the ban would not apply to Muslim members of the military or “people living in the country.”

On Twitter, Christian Broadcasting Network correspondent David Brody wrote that Trump was the only candidate with the “bravery” to call for a Muslim immigration ban, and he predicted it would “give him a boost with evangelicals.”

However, Jonathan Turley, a constitutional law expert at George Washington University, said such a ban “would not only violate international law but do so by embracing open discrimination against one religion. It would make the United States a virtual pariah among nations.”

Muslims make up nearly a quarter of the world’s population, and that portion is projected to hit 30 percent by 2050, at which point Muslims would be nearly as numerous as Christians, according to a Pew Research Center report issued in April. Between 80,000 to 90,000 Muslims immigrate to the United States each year, according to 2011 report by Pew.

Targeted bans on specific groups of immigrants are not unprecedented in U.S. history. In the early 20th century, the United States passed a series of strict quotas on foreign arrivals, culminating in the immigration acts of 1917 and 1924. There were no visas for anyone residing in a designated “Asiatic Barred Zone.” That included most of the Muslim world, from the Arabian peninsula to Indonesia. Those quotas were lifted by the Immigration Act of 1965.

Since last month’s terrorist attacks in Paris, Trump has called for surveillance of mosques and a ban on Syrian refugees, and he has suggested he would consider establishing a database to track Muslims. In the past, Trump has also suggested that Obama was not born in the United States and hinted that the president could be Muslim rather than Christian.

Trump’s standing in most polls has increased as he has made increasingly harsh rhetoric and policies the focus of his campaign, prompting other GOP candidates to follow or ratchet up their own statements. Every Republican presidential candidate has called for a pause in accepting Syrian refugees into the country, with Sen. Ted Cruz (Texas) and former Florida governor Jeb Bush saying Christian refugees should still be allowed in; Trump said at the time it would be impossible to implement because it is difficult to prove a person’s religion.

Most of Trump’s GOP rivals issued statements opposing Trump’s idea. Bush wrote Monday on Twitter that Trump is “unhinged,” while Ohio Gov. John Kasich said the proposed ban “is just more of the outrageous divisiveness that characterizes his every breath.” New Jersey Gov. Chris Christie called it “a ridiculous position,” and Sen. Marco Rubio (Fla.) tweeted: “His habit of making offensive and outlandish statements will not bring Americans together.” Cruz said in an NBC interview that “there are millions of peaceful Muslims around the world.”

Sen. Lindsey Graham of South Carolina said Trump’s escalating rhetoric about Islam endangers U.S. soldiers and diplomats operating in the Muslim world: “The effects of this statement are far-reaching.”

On the Democratic side, front-runner Hillary Clinton and her two main challengers, Sen. Bernie Sanders and former Maryland governor Martin O’Malley all condemned Trump, with O’Malley calling him “a fascist demagogue.”

At the heart of Trump’s statement was a questionable poll commissioned by the Center for Security Policy, a conservative group founded by Frank Gaffney, a veteran of the Reagan administration who accused the Bush administration of helping jihadists infiltrate the government and has been identified as an anti-Muslim extremist by the Southern Poverty Law Center. Gaffney did not respond to a request for comment.

“Mr. Trump is basing his very un-American proposal on the bogus data provided by a well-known and unhinged individual, who’s been rejected by conservatives,” said Suhail Khan, who worked in the George W. Bush administration and has been singled out by Gaffney. “It’s all very un-American. Our country was based on religious freedom. The framers specifically included a ban on religious tests to seek and hold higher office.”

Americans’ fears of Muslim extremists grew last year following a series of high-profile beheadings by the Islamic State, according to a Pew Research Center survey. Half said Islam is more likely than other religions to encourage violence among its believers, the highest in surveys since 2002. Views that Islam encourages violence peaked among white evangelical Protestants, Republicans and seniors ages 65 and older.