Matthew Norman: Nick needs another Dave

The studiedly centrist David Miliband makes a far more natural partner than his brother. With him as Labour leader, Mr Clegg would be a happy self-auctioneer

For all the boldness of his first outing as a gay cowboy flick political analogist, David Davis was misheard picking out the wrong film. As the coalition sheriffs ride off into the sunset for their summer break, the correct reference point isn't Brokeback Mountain but an earlier Western with a subtler and purely tacit homoerotic tinge.

Specifically, with Nick Clegg in mind, it is the closing scene of Butch Cassidy and the Sundance Kid, where the boys gingerly emerge from the house in which they've hid since narrowly fleeing a shoot-out. "For a moment there," says Butch, relishing the serenity of the second, "I thought we were in trouble". The next moment, half the Bolivian army, skulking in the surrounding hills, opens fire.

That May evening he finalised his deal with David Cameron, Butch Clegg must have congratulated himself on a miraculous escape with all of Paul Newman's heroically misplaced sense of relief. If the gunfire hasn't fatally wounded him yet, nor is it raindrops that keep falling on his head. It's hailstones the size of boulders.

How to get himself and his Liberal Democrats out of this siege alive will be the question on his mind as he begins his holiday, and if he spends every waking moment pondering he won't have found an answer by the end of it.

The death of the Lib Dems as a viable solo party was inevitable the moment the coalition was agreed, because a party of protest for disaffected voters of both Right and Left loses all purpose when it makes common cause with the one and terminally alienates the other. With startling speed, so it has proved. Their opinion poll ratings now range from terrifying, at the encouraging end of the spectrum, to does-anyone-know-a-dodgy-private-doctor-in-Knightsbridge-who-doles-out-Vicodin-tables-like-they're-Smarties? at the other.

Under continual guerilla attack from the Liberal Democrat left, where that grinning vulture Simon Hughes encircles menacingly above the fray in the role of deputy leader, and stalked by the Tory right as exemplified by that Barry Norman of libertarianism, Mr Davis, Mr Clegg's paramount concern will be the Labour leadership campaign that continues to electrify us all.

This week's prevailing Westminster meme is that he will, a year or two from now, be compelled to firm up the coalition into an electoral non-aggression pact – the parties standing as coalition Tories or Lib Dems, perhaps, with one fielding no candidate in seats held by the other – or even a full-blown merger. A glance at those recent polls explains this thinking. If the current Lib Dem average of about 15 per cent were replicated at an election, even the AV voting system (and winning the referendum is no gimme) wouldn't protect them from virtual annihilation.

The first clear sign that Mr Clegg is starting to panic, as well he might, came with last week's reference to the Iraq war as "illegal". Too canny an operator to blurt out such a thing under the stress of a PMQs debut, he was making a premeditated stab at re-establishing the Lib Dem brand by reminding us that on this they differ fundamentally from their Tory overlords. This was futile, because public fatigue with Iraq has neutralised it as a political issue (sensational revelations stream from Chilcot almost daily, and no one pays much notice), but you appreciate the desperation. There is no domestic issue, after all, on which he could create a distinct Lib Dem identity without taking a wrecking ball to the flimsy foundations of political expediency on which the coalition was hurriedly built.

Viewed as closet Tories by Labour voters and as the Judas party by 40 per cent of those who voted for them in May, the Liberal Democrats future now rests solely on continually selling themselves to the highest bidder. This is why the Labour leadership is so crucial to Mr Clegg. Soon he will need to start playing Mr Cameron off against whoever is in charge of the opposition, and he will be praying that it's David Miliband.

If it's Ed Miliband, it will, aptly given Mr Clegg's genetic heritage, be a Dutch auction. He would ask for the earth, but in a buyers' market be forced to keep lowering his price. Having won the leadership from the statist Labour left, Mili the Younger would have no option but to strike a hard bargain with the defenders of spending cuts that may, should the recent economic growth be sustained, have come to look a touch draconian.

The studiedly centrist David, a stalwart of the Blairite bunker when a permanent "progressive realignment" with the Lib Dems ("the project") was all the rage, makes a far more natural and amenable partner. With him as Labour leader, Mr Clegg would be a happy self-auctioneer, confident of repeating May's trick by using Mili the Elder to force Mr Cameron to pay a steeper price than he would wish.

The sadness is that there is no survivalist route other than via this death-defying tightrope walk across the chasm into which parties without a raison d'etre tend to vanish as individual entities, and it isn't anyone's fault. As stated often before, the uniquely horrendous electoral mathematics left Mr Clegg no choice but to deal with Mr Cameron, and he played a moderate hand brilliantly to land a monster pot. Just how monstrous he may not have foreseen at the time, but now he must bluff with a weaker hand for the higher stakes of avoiding extinction.

How the cards will fall no one knows, though it may be possible to make a vague guess in September when Labour picks its leader. All that is certain is that the Liberal Democrats are under no moral or political obligation to stick with the Tories if that equates to suicide; that they will use the threat of rupturing the coalition by switching to Labour as a legitimate blackmail tool; and that what hopes they have of emerging from the next election with more than a pitiful rump of MPs rely less on the electoral system in place than on how skillfully Mr Clegg can prostitute himself.

It will be back-breaking work and he has a mountain to climb, but the only conceivable role in which Butch can evade the squadron of snipers surrounding him is as madame and star performer of the Best Little Whore House In Texas.