When is the last time you heard good news coming out of Brussels? Between the Greek debt crisis and a resurgent Russia, ISIS-inspired attacks and an influx of refugees, it has been a rough few years for the European Union. Now many in the United Kingdom want out. Following marathon negotiations with European leaders in February, Prime Minister David Cameron, fulfilling a campaign promise, announced a referendum, scheduled for June 23, to decide the future of the country’s E.U. membership.

Mr. Cameron called for the vote only after winning a number of concessions from Brussels, including limits on the benefits that must be paid to E.U. migrant workers; fair treatment for countries that opt out of the euro; and, most significant, assurance that “references to ever-closer union do not apply to the United Kingdom.” While the prime minister is now campaigning to remain within that union, a number of his fellow Tories support the so-called Brexit, and the British public is almost evenly split on the question.

Many of the challenges facing Europe—from migration to climate change—do require a unified response. But E.U. leaders cannot combat growing euroskepticism, not just in Britain but in countries from Greece to Germany, by simply dismissing nationalist sentiment as a relic of the last century. If the future is not to be “ever-closer union,” E.U. defenders need to formulate a new vision for the European project—and they should not wait until June 23 to do so.