Presumptive Republican nominee Donald Trump's success in the United States is not unique. Europe is being buffeted by similar populist currents.

The United Kingdom might vote to exit the European Union later this month. Moreover, a "yes" victory might spark what John Gillingham of the Harvard Center for European Studies and Marian Tupy of the Cato Institute called a "rush for the exits." An EU breakup would be possible.

The most important question for U.K. voters is: Does belonging improve their lives?