Matthew Norman: Now Clegg must move in for the kill

The Lib Dem leader is perceived to be vulnerable on foreign affairs. But my hunch is that this is his moment of maximum opportunity

With this election, ignorance truly is bliss. Not only does nobody have a clue how it will evolve, but for once no one even pretends to have a clue. And it's wonderful. The combat sport of politics has become hugely exciting, and that's change we can all believe in.

More profound change now seems inevitable. In fact it has already begun. Pick your cliché at will, but whether it's closing stable doors after an equine escape bid or reinserting stoppers in genie-free bottles, there can be no going back however things pan out on and after May. For me at least, the transformational moment came not with the first TV debate last Thursday – perfect catalyst though it was – but a day later, when a poll first established how Labour could win the most seats with the lowest share of the vote, and so form a government. Even this dozy old land, you felt, would no longer tolerate an electoral system of such self-satirising derangement that it might have flowed from the pen of Joseph Heller.

For David Cameron, the Catch 22 is surreal enough to qualify as a Catch 44. He is the party leader who faces the prospect of coming first in the popular vote, second in seats, and nowhere when it comes to the distribution of power. And yet, terrified of a centre-left alliance locking the Conservatives out for generations, he is the only party leader pledged to protect the very system that threatens to ruin him. There can't have been a paradox like it since Prisoners of Bernard Matthews For Justice petitioned Lambeth Palace to re-schedule Christmas for early October. For a man promoting himself as an agent of change, this is not good. Change By Keeping The Old Crap That Drove us To This Desperate Need For Change ... it's not a brilliant slogan, is it? Fighting For The Great Ignored By Being Ignored isn't great either.

One man no one is ignoring is Nick Clegg, whom I'd like to unearth as my protégé at Westminster School 30 years ago. Being a few years older, I don't think I ever actually met him. But I didn't get where I am today by treating a passing bandwagon as one of The Great Ignored, so I'm putting it about that: a) 13-year-old Nick was placed in the boarding house of which, briefly and inexplicably, I was head; b) he was my toast fag (why let Louis Theroux have all the fun?); c) I beat him once for burning the Mother's Pride; d) he sobbed his heart out in Dutch with a heavy Russian accent; and e) this act of character-building brutality that made him the man he is today.

If the above is total fiction, may I say in my defence that I sincerely believed every word at the time of writing? Which brings us, by way of the 2011 Sony Award winner for Most Clunking Local Radio Segue, to tomorrow's debate on Iraq and other areas of foreign policy. Many Lib Dem supporters will, like me, be in a bit of a state about it. The despair we can handle. It's the hope that makes us nauseous with fear.

The closest thing to a consensus amid all this electrifying uncertainty is that raised expectations and perceived vulnerability on foreign affairs make this Mr Clegg's moment of maximum danger. My hunch is that it is his moment of maximum opportunity. If he resists the instinct to defend ground already gained, and goes for Mr Cameron's throat, he can cement and even enhance the recent surge.

While Gordon will lob in whatever flimsy self-justifications, and finely calibrated tactical weapons, pass for his political philosophy, he will be an irrelevant bystander. This is now what Nick, in one of his 27 languages, would call mano a mano. When Mr Cameron sets out to destroy him by painting him as a dangerous naïf on foreign affairs, Mr Clegg must return fire.

With all four big issues, Mr Clegg can turn the tables by using them to illustrate why the Tories are trapped irredeemably in the past. Iraq is a banker, and he'll turn the dial twiddlers' yellow line into the Himalayas by goading Mr Cameron into reiterating his support for that. So is Afghanistan – not a blatantly wicked war in itself, but a deeply unpopular one of greater futility. If the Lib Dem position on troop withdrawal doesn't go far or fast enough, it goes faster and farther than any other.

But it's not enough to hammer Mr Cameron for continuing to call it wrong on both counts. Mr Clegg needs to interweave his support for these wars with his passion for recommissioning Trident into a foreign affairs tapestry barely less outmoded than the Bayeux. The age of using wars and a swanky nuclear arsenal to keep the Security Council permanent seat and justify the play-acting at great power status is over.

Seeking to sustain the pretence that isn't, Mr Clegg should say, is as delusional as seeking to protect this cuckoo bananas system of electing governments. Those who do so belong in the psycho-geriatric ward of political thinking. They have perfect recall of every tiny detail from aeons ago, when Britain was an imperial power. But they cannot remember anything about that accident five minutes ago when she thought she still was but wasn't. The world has moved on. If Mr Cameron prefers to let it go without him, and pass his life in nostalgic daydreaming, that's his business.

With Europe, which is trickier, Mr Clegg must stay on the offensive. When Mr Cameron attacks him as an open-legged Brussels groupie, he should counterstrike by sampling Ronald Reagan with a weary "There you go again". No one is talking about joining the euro now, or will (with Greece in mind) for ages, and a referendum that would surely result in a No vote would be required if ever they are. By banging on about this dead issue, he should say, Mr Cameron betrays how he is still trapped in the Thatcher/ Major civil war, when cricket bats were sabotaged and mad folk in boating jackets reckoned John Redwood was the answer. Mr Cameron, as much as Gordon himself, is yesterday's man.

Whatever the issue, this is his most potent line. It isn't a thoughtful or elegant one, but it works. It also happens to be the truth, which often works too. The Tory leader, clever as he is and for all the glitzy talk of the Big Society, has been re-cast as an agent of mange, and flea-ridden old mutts cannot learn new tricks. The man driven by instinct to fight to protect the system that might cheat him of power is exactly what it says on the tin. He is irrevocably, unimpeachably and immutably conservative.

If the polls have stabilised and the flow of blood staunched, David Cameron is still there for the taking. Has Nick Clegg the balls, mind and killer instinct to take him? A moment of truth is at hand. By tomorrow night we will know something at last, and it might be even more blissful than the ignorance.