Of course, Robertson's - and the other bigots' - fantasies are denied by the wisdom of our Founders as found in the First Amendment, and most precisely on point, Article VI, paragraph 3.
On a secondary level he blamed the acceptance and promotion of decadent behavior which may remove God's protection from our country. Right or wrong, that is his view.
But, vote for people who promote the religion of Islam if that is your view and if that is the direction you want the country to head.