A couple of weeks ago, when I was preparing this post for our Pinning Down Miliband series, I came across this line in a Labour-commissioned report:

“But the Tory-led Government’s wider economic failure has meant they are now forecast to borrow almost £200 billion more than planned in 2010 over the Parliament.”

You can see what they’ve done: compared the borrowing forecasts that the Office for Budget Responsibility first dreamt up with more recent ones. (As we’ve done on ConHome.) The difference, it’s true, is more or less £200 billion. No doubt that figure, or an updated version, will crop up a lot in next year’s election campaign.

Yet, as we know, this is a particularly brazen attack for Labour to make. Messrs. Miliband and Balls may reckon that, by spending more, they would have borrowed less than the Coalition – but they seem to be the only ones with any courage in that particular conviction. The Institute for Fiscal Studies, among others, thinks that a Labour government would have stumbled much further into the red than George Osborne has.

But there’s another counter to this particular Labour attack. It emerges from the borrowing forecasts that they themselves made when it Government. Here’s a graph I’ve put together, comparing expectations at the start of New Labour’s final term with the actual outcomes:

Budget 2005 borrowing forecasts

Which is to say, Labour borrowed £234 billion more than they expected in 2005. £234 billion.

Okay, okay, you say that’s cheating – a lot of the extra borrowing in that period is the due to the financial crisis and the attendant recession. But that’s partially my point: the economy doesn’t tend to operate in line with forecasters’ expectations.

Besides, this is what the graph looks like for the previous term:

Budget 2001 borrowing forecasts

So, between 2001 and 2005, Labour borrowed £90 billion more than they initially forecast. And that’s in only four financial years. With a growing economy.

This isn’t to let Osborne off the hook, but it is to demonstrate two points: i) Labour are being hypocritical (natch), and ii) when it comes to the economy, no-one really knows anything. That’s why I’ve become increasingly sceptical about long-term fiscal rules. We should be wary whenever politicians talk in certainties – particularly if those politicians are Eds Miliband and Balls.