US President Donald Trump has suspended trade talks with Canada and announced an additional 10% tariff on Canadian goods over an Ontario-funded television ad that used audio of former President Ronald Reagan denouncing tariffs. The episode is telling. Yet again, Trump is pretending to defend the Republican Party’s longstanding economic principles while pursuing policies that constitute a wholesale rejection of them.

Many of Trump’s supporters welcome this departure, but they should be careful what they wish for. History is littered with examples of Trump-style populism leading to hi...