Admit it, Clegg, you're in love. You rise each morning with that ache of uncertainty in your breast. You choose that tie, that suit, those shoes with him in mind. You scurry early to the office, practising the phrase that will please him, the gesture he will notice. When you first see him in the corridor … you can't help it. The knees go. He is adorable.

The trouble is, an angry wife and family are watching and waiting back home. This weekend, the Liberal Democrat conference will want to know exactly what has been going on between Nick Clegg and David Cameron these past five months. The party has been forgiving, so far, since Clegg has given its members a high profile, a tinge, a hint of real power. But for how much longer?

From his first storming election debate last April to his present high poll rating, Clegg has led the Liberal Democrats into high places and tempted them beyond their dreams. Assorted party wonkery about income tax thresholds, pupil premiums and electoral reform are suddenly taken seriously. They are in play.

Then there are the jobs. Nineteen out of 57 new MPs have government employment, five of them in the cabinet. No Lib Dem MP ever imagined they would experience such titles, salaries, cars and dispatch boxes. Despite having five fewer MPs than in the old parliament, Clegg has taken his followers where no Liberal since Lloyd George has dared to tread.

So much for the good news. The truth is that behind the razzmatazz Cameron's coalition agreement was a political coup worthy of Walpole. He bought himself a Commons majority for the duration of a parliament. He knew that joining a coalition would devastate the Liberal Democrats at the polls, so he hired enough MPs, bound hand and foot, to give himself safety. The MPs took the bait. Sooner or later, the Lib Dem backbenchers are certain to run for opposition cover, but for those who took jobs with Clegg and entered the coalition, the much-trumpeted "programme for government" last May was short-term glory but a longer-term suicide note.

The question now is, when will the Lib Dem run for cover start? The party's support at the polls has collapsed from 23% to 15% since the election, and can only go further down. This is the classic centre-party squeeze. Extremist coalition partners have no trouble: they negotiate terms, stand aloof and await delivery. Centrist ones must support a general programme which they can only accept or reject table d'hôte. They either back the government of the day, or they attack it. There are no ifs and buts. This is the politics of power, not a thinktank conundrum.

The Liberal paradox remains what it has been since the party's retreat in the face of Labour a century ago. All hope of influence depends on a hung parliament. Yet even this is power for an hour. Once the Liberals have chosen which party to support, they are worse than powerless. They are castigated for supporting it. The Liberal experience in the 1920s and 1930s is the best guide, not vague parallels with coalitions in Germany, Belgium, Italy or the Netherlands.

The last serious Liberal bid for influence was Lloyd George's in 1931, involving a series of "national emergency" deals with Ramsay MacDonald and then the Tories, not dissimilar to those recently negotiated by Clegg and his colleagues. Within a year, the Liberals had split two and then three ways, with Lloyd George, the "Simonites" and "Samuelites" drifting in and out of deals with the Tories for almost a decade. The smoke-filled rooms of Westminster were echoed by local constituency "coupon" deals, with agreements by Liberals and Tories not to run against each other. Local Liberal parties were riven by faction and the Tories always emerged better off. By 1951 there was only one Liberal MP free of Tory support left in parliament. The politics of coalition had served the careers of a few party leaders, but had nearly wiped out the party.

Assuming the present coalition survives its self-allotted five-year term, which is doubtful, Clegg's best hope might seem to be to negotiate a similar "national government" with Cameron's Tories. But this would require a degree of central control over local candidate selection that is hard to imagine. More likely is a rump of senior Lib Dems becoming de facto Tories for the rest of their ministerial careers.

While Cameron might be able to plead with some of his local parties not to run candidates against Lib Dem ministers, Clegg would have a tough time getting his own party to reciprocate. With cuts biting deep into every community and Labour on the rampage, many if not most Lib Dems would rebel. The visibly miserable Vince Cable would resign in a huff and, as in the 1930s, Liberal Democracy would degenerate into Clegg-ites versus Cable-ites, Kennedy-ites or Hughes-ites. Few Lib Dems would gladly hand opposition to the present regime gratis to Labour. It is more than political flesh and blood can stand.

Already this weekend, Lib Dem party workers are asking what they are supposed to say to voters. A Westminster spin doctor might cobble together a line on how far Clegg has "nudged" the coalition on taxation or welfare or a (probably lost) referendum on voting reform. But Clegg's call to arms in yesterday's Times was pure Toryism. There was no room for manoeuvre. To the doorstep question, "Is the Liberal Democrat party for or against the coalition and its cuts?" there can be only one answer. It is for. In which case why are Liberal Democrats still intending to stand against Tories and bring the house down?

We are back to the 1930s and 1950s, to a Tory ascendancy briefly helped by the remnants of a once-popular Liberalism. As long there is a Wilson or a Blair standing ready to render Labour even remotely electable, Liberals only do well when Tories do badly. Having aided the Tories in visiting on the nation the most draconian cuts of recent times, they can expect to be punished mercilessly, while the man who led them to this pass will not be forgiven.

Clegg the politician was a nice chap. He could have made a good departmental minister. Back in May he could have decided otherwise, standing aloof from office and declaring that his party would debate and vote on each government measure on its merits. That would have been a true Liberal Democrat dawn, from which he could have returned perhaps more successfully to fight the Tories at the polls.

Instead Clegg chose glory in death. He came to the rescue of Cameron in the latter's hour of need and enabled him to steer the economy out of Labour's morass. I have no doubt Cameron will show gratitude, offering honorary life membership of the Tory party, a safe seat and a comfy place on the Notting Hill sofa. For Clegg that might be enough. But as leader of the Liberal Democrats, he has booked a ticket to oblivion.