Here’s how The Economist’s Bagehot characterised the performances of Gordon Brown and David Cameron at their respective press conferences this week:

On Gordon Brown: “… [he] was his usual funereal self (even if he did manage a decent joke about the date of the general election). I thought he looked exhausted. But what he had to say was relatively upbeat: the recession is over; the government has plans for the “job-rich prosperity” that is just around the corner and an expanded middle class.

On David Cameron: “… [he] was his usual breezy self, cracking jokes, remembering journalists’ names, etc. But what he actually had to say was pretty miserable. Britain is broken; inequality is at a record high; this is a dreadful country in which to bring up children; our public finances are heading the same way as Greece’s.

Bagehot concludes:

This is the choice the nation faces: between a glum optimist and a perky doom-monger. The one big thing the press conferences had in common was that both men ducked questions about how exactly, or even inexactly, they would cut public spending.

It’s an acute observation. Both Mr Brown and Mr Cameron are playing against type. The Prime Minister was at his best, most credible, when cast in the role of granite-faced iron Chancellor. The Tory leader was at his best, most authentic, when portraying a fresh-faced sunny face to the world.

But now both are trying to prove they have what it takes to be Prime Minister by camouflaging their natural dispositions: Mr Brown is trying to look cheery and optimistic; while Mr Cameron is trying not to. It suits neither of them especially well.

So where does that leave Nick Clegg? Well Nick, it strikes me, is trying to strike a balance: a leader who’s impatient – even angry – with the Westminster systems, and years to change things for the better.

In effect, he’s a “doom-mongering optimist”.

This tone can be highly effective, and certainly can help him connect with the public’s anti-politician mindset. But there are two risks with this strategy.

First risk: Nick risks people primarily seeing him as angry. Nearly every statement from Nick – and nearly every intervention at Prime Minister’s Questions – begins by saying how Labour has failed and the Tories would be no better. Which is fair enough. But by the time he starts talking about what the Lib Dems would do positively, many people will have already formed a vivid impression that Nick is an angry oppositionist. And that’s a shame because I don’t think that’s what Nick is.

Second risk: that Nick’s intelligent, questioning optimism is buried. One of Nick’s most attractive qualities is that you can tell he’s never quite satisfied with his own answer; that he realises politics is about continual progress, improving on what’s gone before, not pretending you’ve got a silver-bullet solution. It’s an open, honest and questing approach to politics to which I think people react well. But of course it’s hard to encapsulate in a 10-second vox pop.

The televised leaders’ debates (assuming they do still happen) will be Nick’s first major opportunity to project an image of himself direct into the nation’s front room. He needs to set himself apart from the Labour and Tory leaders, and in particular from Mr Cameron with whom there are some superficial similarities.

But there’s no point trying to ‘out-doom’ the Tories about how bad Britain is; nor in trying to pretend, like Labour, that everything’s rosy in the garden. What we need Nick to convey – and what I think chimes with his natural personality, and with the values of the Lib Dems – is a sense of “honest optimism”.

That’s all there is to it … now over to you, Nick