Declaration of independence?
The issues driving the debate are basically national independence vs. economic integration. "Leave" supporters argue that as the EU becomes more centralized, its interference in Britain's internal affairs is becoming excessive, notably the fact that Britain no longer has much control over immigration from other EU countries because the EU is committed to free internal movement of people (although this cuts both ways -- there are more British people living in other EU countries than vice-versa). They also note that the real power in the EU resides in institutions which are not democratically elected, so that democracy is being traded for oligarchy. Finally, the EU is becoming a German-dominated superstate -- the very thing Britain fought both world wars to prevent. Against all this, the "Remain" camp argues that the economic benefits of European integration are worth it, and that separating from the EU would leave Britain isolated and marginalized.
If the Remain side wins, the option of "Brexit" (short for "British exit") will be off the table permanently, or at least for as long as the EU exists. A Remain vote would be viewed as settling this long-contentious question for a generation or more. And since younger people generally favor Remain, while Leave supporters tend to be older, the Remain tendency will probably grow over time.
If the Leave side wins, the country will be committed to a complex process of negotiation to disentangle itself from the snarl of political, economic, and legal connections which have developed between itself and the rest of the EU. Most of this will involve not severing those connections but restoring them to a conventional international-relations footing as opposed to the current relationship of metropolis (Brussels) and province. Nor will the process be one the EU can take lightly. The UK is the EU's second-largest economy and its third-largest member state by population; it also includes London, which is by far the most important financial-services center. It is a major market for other European countries' exports. Contrary to the Remain camp's predictions of disastrous ostracism, the UK would have a great deal of leverage to win favorable terms of departure. (One might also note that three western European countries -- Norway, Iceland, and Switzerland -- are not and never have been in the EU, and they are doing at least as well as comparable EU states.)
A British Leave vote would resound across the whole problem-wracked conglomeration. It would likely lead to intensified calls for similar referenda in (at least) Denmark, Sweden, and the Netherlands. French nationalist leader Marine Le Pen has called the British referendum "a key moment in European history", and her National Front party favors a similar referendum in France. Brexit followed by Frexit would be the end of the EU in anything like its current form, and the EU is even less popular in France than in the UK (only 38% of the French view it favorably, compared with 44% of the British). Indeed, over the last year, support for the EU has fallen sharply in most of the member states, even in Germany itself.
The unraveling could eventually go further. The Irish Republic is culturally and economically more strongly connected to the UK than to any other EU state. And just as Britain's linguistic, cultural, and historical ties with the US and other English-speaking nations offer the permanent temptation of a "family" alternative to the EU, so the similar ties linking Spain and Portugal with dynamic and growing Latin America may eventually seem more attractive to those countries than the endless stagnation they suffer under the EU's austerity regime.
Within Britain itself, a Leave victory could re-open the question of Scottish independence, supposedly settled by the vote there less than two years ago. Scotland is more pro-EU than the rest of the UK, and the Scottish Nationalist party has made it clear that leaving the EU would provoke a second referendum on Scotland's status. (In Wales, by contrast, opinion on the EU is evenly split.)
Whatever the consequences, Brexit is looking more likely. During most of the campaign, polls showed the contest almost too close to call, but in recent days the Leave side has opened up a 10-point lead. This seems startling given that the whole political establishment -- most leaders of the two biggest parties, along with most of the media -- have strongly supported the Remain side. However, this unanimity has enabled the Leave campaign to frame the debate as the people vs. the elites, which resonates with a population long frustrated with the establishment's unresponsiveness on issues such as immigration.
Whichever side wins the referendum, it will be for the best if it wins by a large majority. Such a momentous decision should not depend on a narrow margin, and the people's verdict should be clear enough that its legitimacy and decisive character are beyond question.
The British will make their decision next Thursday. It will resound across Europe and beyond.