The race for the 2016 Republican presidential nomination is finally beginning to look a bit more conventional. After weeks in which the leading candidates, Donald Trump and Ben Carson, appeared able to flout all the rules of politics, Carson’s support appears to be declining as many had expected. Since the end of October, the former surgeon’s national support has declined 35 percent. In Iowa—a state in which evangelical Republicans have a much-larger presence than others and where Carson has polled best—Carson has relinquished first place…