If countries leave the eurozone, this will undermine confidence in the British political establishment. It is true that the establishment was divided on the euro. But even those opposed to it subconsciously assumed that European integration was an ongoing process somehow in tune with the logic of history. That assumption is now being destroyed.
It is difficult to think of issues of comparable magnitude in Britain's 20th century history. But in 1931 the crisis of austerity caused by the gold standard led to the collapse of the second Labour government, which could not agree on whether to cut unemployment benefit. The result was the formation of a national government and a realignment of parties.