Imagine the scene. Britain’s Prime Minister is sitting in his office in Downing Street. He is nervous about Britain’s links with the rest of Europe. His party is divided: he wants Britain to remain a member of the club, but many of his backbenchers and some Cabinet members want out. To keep his party together he has promised to renegotiate Britain’s terms of membership and to put the outcome to a referendum. However, the polls show that a clear majority want Britain to “leave Europe”. A tactical exercise in party management threatens to become a strategic nightmare.

We have been here before. That scene might come true in the next two or three years; but it is also a description of the problem Harold Wilson faced in 1974. Edward Heath’s Conservative government had taken Britain into the Common Market (as it then was) the year before. Wilson’s Labour Party was bitterly divided on the issue. In the February 1974 general election, Wilson promised a referendum. Labour squeaked to a narrow victory, and improved its position slightly in a second election in October 1974. Now Wilson had to keep his promise.

Early in 1975, Wilson returned from Brussels, hailing his renegotiations as a triumph for Britain. He said that, given the new terms, he would strongly recommend a “yes to Europe” vote in the referendum, to be held that June. In fact the negotiations had achieved little (as was evident from an internal Labour party analysis, leaked to me – then a young journalist on the Sunday Times – much to Wilson’s irritation). However, most voters were disinclined to take the risks of leaving the Common Market. Polls tracked the sharp change in the public mood, which culminated in a two-to-one majority for “staying in Europe” in the June referendum.

If a new referendum were held in the next few years, would history repeat itself? It depends on whether the similarities or the differences with 1975 end up mattering more.

Similarities. Assuming that David Cameron calls the referendum with the aim of securing a “yes” majority, we would have the leaders of all three main parties campaigning to keep Britain in the EU. The issue would move from the back of most voters’ minds to the front. Many voters who are generally hostile to “abroad” would be faced with the hazards of Britain going it alone. Happy to tell pollsters at other times that they liked the idea of telling Brussels to get lost, they would find themselves confronting the practical dangers of withdrawal. As so often happens in referendums round the world, the status quo would become increasingly attractive as decision day approached. “Yes” would win the day.

Differences. In 1975, not a single major newspaper supported withdrawal – the tiny communist daily paper, Morning Star, was alone in wanting Britain out of Europe’s “capitalist club”. As far as prominent British politicians were concerned, almost all the people that commanded respect advocated a “yes” vote. (On one occasion, Margaret Thatcher, the Conservatives’ new leader, sported a jumper displaying the flags of the Common Market’s members, to demonstrate her devotion to Europe.) The supporters of a “no” vote were those depicted by virtually all the papers as extreme – Tony Benn and sundry trade union leaders on the Left, Enoch Powell and Ian Paisley on the Right.

This time, there would be plenty of newspapers and “respectable” politicians advocating withdrawal. And whereas the decision in 1975 could plausibly be depicted as simply about economics, markets and jobs, a “yes” vote this time would far more readily be presented by the “no to Europe” campaign as a step towards the hated destination of political integration.

Which would end up having more power if there were a referendum this time: the similarities or the differences? YouGov research suggests that British voters can be divided into three broad categories. The largest, though not a majority, are “worried nationalists”. Their dislike of “Europe”