Buchanan understands Trump very well, because Trumpism is a kind of Pat Buchananism reborn – an anti-free trade, protectionist, anti-open borders paleoconservatism.
Yes, there are plenty of toxic aspects to Trumpism and Buchananism, and Trump seems to want a more militaristic foreign policy than Buchanan did (without going to the neocon extreme), but at the heart of Trumpism is: the failure of neoliberalism in America. Part of that failed neoliberalism was open borders and mass immigration, not just free trade, deregulation and the abandonment of fiscal policy. Trumpism is a kind of anti-neoliberal conservatism, and Trump is a “Bernie” for right-wing people. Left-wing people should take careful note of this.