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Since the Cold War ended, U.S. politics has seen a series of insurgent candidacies. Pat Buchanan prefigured Trump in the Republican contests of 1992 and 1996. Ralph Nader challenged the Clinton wing of the Democratic Party from the outside in 2000. Ron Paul vexed establishment Republicans John McCain and Mitt Romney in 2008 and 2012. And this year, Trump was not the only candidate to confound his party’s elite: Bernie Sanders harried Hillary Clinton right up to the Democratic convention. What do these insurgents have…