AMERICAN POLITICS has long been characterised as an iron triangle. It consisted of interest groups with narrow concerns, an executive branch trying to keep the groups’ ambitions in check, and a Congress entangled in a constant quid pro quo with the groups. But a new Republican iron triangle entails not interest groups but tribal ones. Primary voters, politicians and infotainment pundits have created a feedback loop of extremism, outrage and fear that is ripping American politics apart.