Is it the breadth of their agenda? Not really, look at some of the new campaigning outfits, like the irritating 38 Degrees.
Is it that they are standing for elections? Not really, pressure groups can and do so.
Is it that they have to be registered as a political party? Please don't bore me with legal definitions...
No, what makes a political party a political party is that they aspire to run the country, which means that their programme has to cover all the essentials that a government might need. They can't, for example, say - oh, we're not very interested in defence. They can't say that they are just going to assume that somehow everything about economics or business is somehow irrelevant to their great cause.
The trouble is that this is precisely what the Left has been doing over the past generation. They have been doing so for different reasons, but that has been the basic underlying problem.
The Labour Party has been ignoring economics for fear that they will be thought too radical if they mention it at all. The Lib Dems have been ignoring economics because, with a handful of noble exceptions, they can't see what it has to do with Liberalism.
Part of Jeremy Corbyn's attraction seems to me that he has disputed the basic underlying Labour agreement: don't mention business, except to praise the banks.
Because the truth is that no potential government is going to be elected if they don;t set out - in some detail - how they are going to build prosperity in the nation.
The idea that somehow you do so by making a handful of people very rich, and letting the wealth trickle down, as long since been revealed as a major delusion - but the political parties of the Left appear to conspire not to mention this.
They talk about shuffling the deckchairs a bit. They talk a great deal about welfare, and these are not unimportant, but that's not the core issue for government. It is about moving prosperity around, not creating it. Nor is it what people want to hear - a credible programme for the economy that underpins the majority of people's lives.
All they are being offered is the following:
- Business as usual (broken) with Osborne or Blair.
- A tumbledown mixture of stuff on the welfare state (most of the Labour leadership contenders).
- A heady mix of stuff about benefits, Europe, youth services and Trident (the Lib Dem conference agenda this month).
It's fine as far as it goes, but none of this provides a way forward for the Left on economics. Labour and Lib Dems alike have preferred, for some reason, to defer to their Conservative rivals on economics and business. It is hardly surprising, in those circumstances, that - when people feel nervous about the future - they should do the same.
Of course I'm going to the Lib Dem conference in Bournemouth. I wouldn't miss it for the world. But don't let's pretend that this is a debate about any kind of programme for government.
Nothing about employment, the banks, business, enterprise. Nothing about economics, currency reform, money management. Nothing about money at all.
So, while we're there, let's try to make sure that the party tiptoes back onto the economic agenda it has virtually abandoned. Because, if I'm going to be a member of a glorified pressure group, there may be more successful ones out there I could give my money to.
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