Despite his hundreds of rallies, his many interviews and his hyperactive Twitter feed, it’s still really hard to tell the difference between what Donald Trump actually believes, what he says to whip up his crowds, and what he says merely because it popped into his head one afternoon. Not so with Senator Ted Cruz, the arch-right Texan who is one of Mr. Trump’s two remaining Republican opponents. Americans can be sure he believes what he says, and that is pretty terrifying. His calls for carpet bombing Muslims to punish the Islamic State, and more recently for putting Muslim neighborhoods under...