The similarities in the two campaigns suggest powerful shared tensions are tearing at the world's most enduring English-speaking democracies. The forces unleashed seem likely to rattle the respective political systems for years to come.Read MoreThey both feature a torrent of populist anger, fear-mongering by politicians, hostility toward distant elites and bitterness toward the ruling political, media and business establishment.

In the U.S. and the UK, emotion and hyperbole have become a constant undercurrent of politics and are being used as a weapon by politicians in both camps, right and left, "Leave" and "Remain."Brexit: What does the EU referendum mean for the US?Former London mayor Boris Johnson warned in May that the E.U. was the latest manifestation of a 2,000-year project to unify Europe under a single government."Napoleon, Hitler, various people tried this out, and it ends tragically," Johnson told the Sunday Telegraph in arguing for leaving the European Union. "The EU is an attempt to do this by different methods."Such emotive rhetoric is reminiscent of Trump's own claims that the independence of the United States itself is under threat."We will no longer surrender this country or its people to the false song of globalism," Trump said in a major foreign policy speech in Washington in April.