News that membership of the Scottish National Party had surged to almost 90,000 must have caused a degree of angst at the Labour Party, with 190,000 members, and the Conservatives, with 170,000. Both of them have a population 10 times larger to recruit from.

The decline in membership of the two big parties from the millions they had signed up in the postwar years is dramatic.

According to the last census, there are now more people — 176,632 — who say that their religion is Jedi Knight than claim to be Tory Party members. It must say something about the state of British politics when the Tories attract fewer followers than a Hollywood fantasy cult of space travellers from another galaxy.

This dwindling membership — average age 74 — is a major reason for David Cameron’s troubles. The local membership, some 0.25% of the UK population, select the Tory candidates for Parliament.

If you want to be a candidate, it pays to share the members’ traditionalist views on gay marriage, immigration and the European Union. Getting elected requires appealing to the moderate centre, but Cameron’s party and MPs demand the opposite.

So it is not surprising the Prime Minister is struggling to curb his party’s Europhobia. Creating a European policy that could unite the Tory party frustrated Macmillan, undermined Heath, toppled Thatcher and ruined Major. Now it is pushing Cameron, with his promised renegotiation and referendum on EU membership, to take a huge and reckless gamble with Britain’s economic future.

The campaign, let alone the result, could be seriously destabilising. One of the little-noticed side effects of the Scottish referendum was, that as the campaign warmed up, businesses’ desire to invest cooled down.

Investment plans were put on hold while companies waited for the vote. Allianz Global Investors, in a report after the event, identified well over £1 billion of infrastructure investments — from hospitals to a new Aberdeen ring road — which foreign backers had shelved pending the outcome.

Although the EU referendum is not scheduled until 2017, the campaign will start immediately after the next general election if the Conservatives remain in government. Business hates uncertainty; overseas investors worry about it and the political risk is even greater.

If investment into Scotland dried up in the months before their vote, how much more likely is it that investment into the UK will do so in the two years before the EU vote? We should be in no doubt that such a slump would be quite enough to send our slowly recovering economy back into a tailspin.

Is it really worth the risk? What problems does Britain face that are so great leaving the EU would be the solution? The stock answer is immigration. But this is an emotional rather than a rational issue. Study after study shows that immigrants from the EU are better-educated, harder-working and less likely to claim benefits than the local population. We are tens of billions of pounds better off as a result.

It is worth asking too what would happen to the hundreds of thousands of Brits who have retired to Spain, Cyprus and elsewhere in the EU, and the thousands more who own holiday homes. Curbing immigration is a two-way street; if we ban EU nationals from our shores, we should not be surprised if British home owners lose their residency rights abroad.

If we restrict immigrants’ access to benefits and the NHS, we should not be surprised if those lobster-coloured Brits on the Costa del Sol find themselves much less welcome in their local hospitals. Importing young, healthy and hard-working Europeans and exporting a not vastly dissimilar number of elderly, retired and potentially infirm nationals actually leaves the UK distinctly better off.

Attitudes to regulation are almost as emotive. People grossly overestimate how much regulation comes from the EU. They tend to think it all does but a Government-commissioned study covering 1997 to 2009 found only 7% of primary legislation and 14% of secondary legislation originated in Brussels. The rest is home-grown. It also found the UK was successful in lobbying for changes in EU legislation 50% of the time and part-successful 30% of the time.

Most, though admittedly not all, legislation is there to support the single market, because a single market needs minimum common standards.

So it makes no sense to say, as some do, that they want the single market to be completed at the same time as saying they want less regulation. Business needs to note that the Working Time directive, which it would like abolished, is quite popular in the country at large. Families are attached to the idea of paternity leave, and to there being a limit on the number of hours an employer can demand his people work.

Despite the fuss about regulation, the Centre for European Reform says the UK product market is the second-most lightly regulated in the developed world, and there is less employment protection here than anywhere outside North America.

Perhaps that is why, when challenged, most people find it hard to say which specific rules they want abolished or changed.

The brutal fact is that if we want the benefits, we have to accept there will be bits we don’t like. As a former government minister put it: “The EU is like foie gras: if you are really going to enjoy it and appreciate it, it helps not to think too much about how it is made.”