In the statement outlining his proposal, Trump notes two bits of data. Here's a direct quote:
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."
We'll start with the Pew data. Trump doesn't link to it, so it's not clear what exactly he's looking at. The polling firm has found that Muslims across the globe are overwhelmingly opposed to the Islamic State and in 2007 that Muslims were much less likely to view suicide bombings as justified than five years prior. Pew also found a partisan split in which Republican Americans were far more likely to hold negative views of Muslims than Democrats. In 2011, they learned that U.S. Muslims almost never consider suicide bombings to be justified.
Since Trump simply said that "large segments" of the "Muslim population" hate the United States, it's hard to tie that to a number. (If we missed something, by the way, we're happy to add it.) But that second set of data, from the "Center for Security Policy," is more important — and it's possible that Trump simply overlaid those findings onto Pew's estimates of the number of Muslims in the United States.
The Center for Security Policy is an organization run by Frank Gaffney, who is identified as an anti-Muslim extremist by the Southern Poverty Law Center. The survey Trump cites was conducted earlier this year on behalf of the organization.
And now all of the various caveats.
1. This was an online survey of 600 people. The only available information about how the poll was conducted indicates that it was conducted online; Georgetown University's Bridge Initiative has reported that it was conducted using an opt-in Internet survey. That makes it less reliable as a national bellwether than more traditional polling methods where all members of a population have a chance of being selected.
2. The questions asked were agree/disagree, which can favor an "agree" response. Our pollster Scott Clement points to research suggesting that people answering poll questions are more likely to agree than disagree with a provided statement. The key questions — on a) having a choice of being governed by Sharia and b) whether violence against America is justified — were asked in that agree/disagree format, leading more people to say they agree than might be accurate.
3. Many U.S. Muslims are first generation immigrants, who may speak English as a second language. According to Pew, 63 percent of U.S. Muslims were born in another country, suggesting that their ability to read and write English may be limited. In which case a nuanced survey question may be more difficult to navigate.
4. The organization conducting the survey matters. There is no question that the results of the survey — which would certainly bear retesting if accurate — were influenced by the organization that paid for it. The Center for Security Policy likely sought poll numbers showing that a significant number of Muslims were supportive of violence against the United States, and the center got what it paid for.
And now, the ace in the hole:
5. That survey is of U.S. Muslims. Meaning that even this already questionable survey has absolutely no relationship to the people from overseas that Trump hopes to restrict.
There is, in fact, no reliable evidence that a large percentage of Muslims in the United States — or, for that matter, Muslims hoping to travel to the United States — support doing harm to the country or plan to commit acts of violence. Trump has learned over the course of the past few months that railing against Mexican immigrants and Muslim migrants pays political dividends. He, like Gaffney, is happy to seize on questionable numbers to make his point.