President Obama asked the country for tolerance toward Muslims in his speech after the December 2015 terrorist attacks in San Bernardino, CA. As Obama put it, “We cannot turn against one another by letting this fight be defined as a war between America and Islam…[Terrorists] account for a tiny fraction of more than a billion Muslims around the world—including millions of patriotic Muslim Americans who reject their hateful ideology.”Footnote ¹

But this speech was destined to be an exercise in futility. After all, people who disliked Muslims also had very unfavorable opinions on Barack Obama (Kam and Kinder Reference Kam and Kinder2012; Tesler and Sears Reference Tesler and Sears2010). Attitudes about Muslims were one of the strongest predictors of vote choice for president in 2008 and 2012 despite their insignificant influence on the 2004 election. While few voters acknowledged Obama's race as a factor, a majority of Republican voters in 2012 said Obama's religion made them less likely to vote for him—almost none of whom identified Obama as a Christian (Tesler Reference Tesler2016a). That strong link between anti-Muslim sentiments and opposition to Obama helped Islamophobia emerge as a significant predictor of party identification for the first time ever during his presidency (Sides et al. Reference Sides, Tesler and Vavreck2018).

It also provided a mobilized constituency within the Republican Party that was particularly receptive to Donald Trump's explicitly Islamophobic rhetoric and policies. When Trump announced his presidential candidacy in June 2015, less than half of Republicans said they could vote for an otherwise qualified Muslim candidate for president.Footnote ² Most of the GOP also viewed Muslims unfavorably, with Republicans rating them substantially lower than any other religious group in 2014 surveys.Footnote ³ A majority of Republicans continued to believe that President Obama was a foreign-born Muslim (Tesler Reference Tesler2016b)—mistaken beliefs strongly rooted in anti-Muslim sentiments (Kam and Kinder Reference Kam and Kinder2012; Tesler and Sears Reference Tesler and Sears2010).

Donald Trump had rallied these Americans in the past by questioning President Obama's citizenship. Beliefs about Obama's birthplace and religion were both strong correlates of Republicans’ support for Trump in April 2011 when he was the face of the “birther movement” (Tesler Reference Tesler2016b). So, it was not surprising that Trump bucked Obama's call for tolerance after San Bernardino. Instead, he proposed “a complete and total shutdown of Muslims entering the United States” the very next day. This so-called “Muslim Ban” was condemned by prominent Republicans, such as Paul Ryan, Mitch McConnel, and Mike Pence. But Trump's views reflected his party's Islamophobic base. Large majorities of Republican primary voters favored the Muslim Ban in 2016 exit polls.Footnote ⁴

Donald Trump rode that Islamophobic wave throughout the 2016 campaign. Unlike prior Republican primaries, when attitudes about Muslims were unrelated to vote preferences, Trump galvanized his party's most Islamophobic voters (Sides et al. Reference Sides, Tesler and Vavreck2018). His campaign also activated Islamophobia in the general election. Attitudes about Muslims were a stronger predictor of vote choice in 2016 than they were in Obama's elections, with those attitudes significantly predicting party vote changes from 2012 to 2016 (Sides et al. Reference Sides, Tesler and Vavreck2018).

However, Donald Trump's Islamophobic rhetoric also provoked a backlash. Information portraying the Muslim Ban at odds with inclusive elements of American identity made the policy less popular (Collingwood, Lajevardi, and Oskooii, Reference Collingwood, Lajevardi and Oskooiiforthcoming). His historic unpopularity helped make Muslims more popular as well (Telhami Reference Telhami2017). His record-high first-year disapproval ratings were concentrated among the growing share of Americans who rated Muslims favorably.Footnote ⁵

It is clear from these results that attitudes about Islam have become increasingly important in American political behavior. Yet, we still know little about the nature and origins of Islamophobia (though see Kalkan et al. Reference Kalkan, Layman and Uslaner2009). The article in this volume by Lajevardi and Oskooi, therefore, makes an especially important contribution to the growing literature on Islamophobia in American politics by developing different measures of Islamophobic attitudes and showing how those measures predicted support for Donald Trump.