Peter Beinart wrote in May's Atlantic, The New Enemy Within "Why are conservatives more hostile to Muslims and Islam today than they were in the terrifying aftermath of 9/11? And why have American Muslims, who in 2000 mostly voted Republican, apparently replaced gays and feminists as the right’s chief culture-war foe?"
But if conservatives no longer believe they can transform the Middle East, they still greatly fear terrorism by Muslims. A 2014 poll by the Pew Research Center found that Republicans were 31 percentage points more likely than Democrats to be “very concerned” about the threat of “Islamic extremism” around the world. The result is a mismatch between conservative anxieties and conservative methods. Although most conservatives are happy to bomb ISIS, the American right has lost its appetite for a vast overseas struggle against jihadist terror. Instead of tempering their view of the threat, conservatives have domesticated it. By reconceiving the Islamist danger as a largely domestic problem, conservatives can now fight it ferociously without having to invade any other countries.
All they need to do is prevent Muslims from Islamicizing America. Thus, in 2010, Newt Gingrich called the adoption of Sharia “a mortal threat to the survival of freedom in the United States.” To prevent an “attempt to gradually ease Sharia law and the Muslim faith into our government,” the GOP presidential candidate Herman Cain vowed in 2011 not to appoint any Muslims to his Cabinet. In 2012, five Republican members of Congress sent a letter to the State Department’s deputy inspector general suggesting that Hillary Clinton’s aide Huma Abedin, who is Muslim, had influenced the State Department on behalf of the Muslim Brotherhood.
The right’s new focus on the danger that Muslims allegedly pose at home is McCarthyite in a very specific sense. In the late 1940s and early 1950s, conservatives were deeply frightened by communism overseas. But many of them also worried that the Truman administration’s military spending was creating budget deficits and that America’s entrance into NATO (and later the Korean War) was undermining American sovereignty. By hunting alleged communists in the State Department, and thereby suggesting that the real threat lay not overseas but at home, Joseph McCarthy reconciled those concerns. “The reason why we find ourselves in a position of impotency is not because our only powerful potential enemy has sent men to invade our shores,” he declared in his infamous Wheeling, West Virginia, speech in February 1950, “but rather because of the traitorous actions of those who have been treated so well by this nation.” By suggesting that the real Islamic threat lies at home, today’s conservatives are saying much the same thing.
He also cites the conservative fear of "a war on christianity". And since they can't demonize the feminists and gays as much as they did, they blame Muslims. This is probably why the GOP establishment is mad with Trump for bringing the discussion back to Mexicans. They already had someone to blame, now they're stuck demonizing the biggest growing segment of the voting population.