In response, Trump characterized Khizr Khan as "emotional" and speculated that Ghazala Khan didn't speak at the convention because "she wasn’t allowed to have anything to say," an apparent reference to the family's Muslim faith. Longtime Trump confidant Roger Stone recently took to Twitter to allege that Khizr Khan is in fact an agent of the Muslim Brotherhood.
For many Muslim Americans, these types of groundless suspicions — of their religion, or of their loyalty to the country — have become a familiar fact of living in the United States in the post-9/11 era. Public opinion polling has generally shown that substantial minorities, and in some cases majorities, of American citizens support treating Muslims differently than other religious groups, or otherwise hold negative views of Islam.
Here's a rundown of what the latest polling says.
1. A majority of Americans support Trump's Muslim travel ban.
As of June, half of all Americans supported Donald Trump's unconstitutional proposal to temporarily ban Muslims from entering the United States. Support for the ban ticked upward after the shooting at an Orlando nightclub in June in which the shooter claimed allegiance to the Islamic State.
It's worth noting, however, that responses to this survey question have been highly volatile and seem to be heavily influenced by how the question is worded. The NBC/SurveyMonkey poll cited above asked respondents "How do you feel about temporarily banning Muslims who are not U.S. citizens from entering the U.S.?" Monmouth University asked registered voters if they "support or oppose banning all Muslims from entering the U.S.?" at roughly the same time. Only 21 percent of voters said they supported such a ban in that polls.
2. Americans view Muslims more negatively than members of any other religion.
Fully one-quarter of Americans say they hold a "mostly unfavorable" or "very unfavorable" opinion of American Muslims, according to a recent NBC News/Wall Street Journal poll.
That poll didn't ask about views of other religious groups, but other recent surveys have. In 2014, the Pew Research Center asked Americans to rate their feelings toward various religious groups on a 0 to 100 scale, with 0 representing the most negative feelings and 100 the most positive. Americans placed Islam at the bottom of the scale, with a mean rating of 40 out of 100, just a hair behind atheists.
3. Over 40 percent of Americans admit to feeling prejudice toward Muslims.
A 2010 Gallup poll found that 43 percent of Americans said they felt at least a little prejudice toward Muslims. Nearly 1 in 10 admitted to a "great deal" of prejudice. These numbers were considerably higher than for any other religious group.
4. One-quarter of Americans think a special ID card for Muslims would prevent terrorist attacks.
A June 2014 Gallup poll found that 25 percent of American adults say that "requiring Muslims, including those who are U.S. citizens, to carry a special ID" would be an effective way to prevent terror incidents like the Orlando nightclub shooting.
5. One-third of Americans want the government to keep a closer eye on Muslim citizens.
A March 2016 Pew poll found that one-third of American voters — including nearly two-thirds of Trump supporters — say that U.S. Muslims should be "subject to more scrutiny" solely because of their religion.
6. Half of Americans say that some or all American Muslims are anti-American.
And according to a February 2016 Pew Research Center poll, more than one in 10 Americans say that "most" or "almost all" American Muslims harbor anti-American views. Among conservative Republicans, that figure is nearly one in five.
7. Nearly half of Republicans say Islam encourages violence.
A December 2015 Quinnipiac poll found that 28 percent of all Americans said that "mainstream Islam encourages violence against non-Muslims." Among Republicans, that figure was 47 percent. Only 13 percent of Democrats said the same.
8. Most Americans say Muslims haven't done enough to oppose extremism.
A December 2015 survey by the Public Religion Research Institute found that over half of Americans said that "American Muslims have not done enough to oppose extremism in their own communities."
While Muslims faced heightened scrutiny and suspicion on account of their religious beliefs, the way they live their lives in many ways mirrors that of other Americans. One of the best available surveys of American Muslims was conducted by the Pew Research Center, which interviewed over 1,000 Muslim American adults in 2011.
"Muslim Americans look similar to the rest of the public" on a variety of issues, the study found. "Comparable percentages say they watch entertainment television, follow professional or college sports, recycle household materials, and play video games. About one in three say they have worked with other people from their neighborhood to fix a problem or improve a condition in their community in the past 12 months, compared with 38 percent of the general public."
When asked how they thought of themselves — whether as a Muslim first, or as an American first — about half said Muslim first, similar to the percentage of Christians who see themselves as Christian first.
And despite the challenges of life as a Muslim in the post-9/11 era, American Muslims stand out for a strong sense of optimism about life in America. As of 2011, Muslims were about twice as satisfied (56 percent) as the general public (23 percent) with the way things were going in the U.S.
A majority of Muslim Americans (56 percent) said that most Muslims simply want to adapt the ways and customs of American life, as opposed to remaining distinct from American society at large. Tellingly, only 33 percent of the general public thought that most Muslims want to adapt the ways and customs of American life.