This autumn conference was Nick Clegg's third since leading the Liberal Democrats into Coalition and each has followed a familiar script. The opening is preceded by talk of the hard time he is going to have, courtesy of a discontented and disillusioned party rank and file, and the challenge he could face to his authority. And at each, Mr Clegg has weathered – with a certain grace and good humour – what turned out, in the event, to be an almost non-existent storm. This year was no exception.

The tasks before the Liberal Democrat leader at Brighton were as clear as they have always been, and as unenviable. He had to argue that, even as the junior partner in the Coalition, the party had not squandered its time in government. He had to show that there are advantages as well as the more obvious electoral disadvantages to holding even a small share of power. But most of all, he had to demonstrate that, after almost two and a half years supping with a Conservative-led Cabinet, the party has not sold its soul.

This may well have been the true purpose behind the apology – for breaking his election promise on university tuition fees – that Mr Clegg issued as a perverse taster for the conference. It was a decision whose timing (just as memory of the broken promise seemed to be starting to fade) and content (he was sorry for breaking the promise, but not for making a promise he had no prospect of keeping) were as dubious as each other. But a bizarre turn of events – the genius of a satirist who set the apology to music – allowed Mr Clegg to snatch a very Lib Dem sort of victory from defeat. And his response to the gag – self-deprecating amusement and instructions that any proceeds go to a local charity – was a model of how to deal with such things.

While in the short term Mr Clegg salvaged more than seemed remotely possible, however, the underlying travesty will not go away. It will dog Mr Clegg for as long as he leads the Liberal Democrats. But he is also right in that this promise and its fate exemplify the political liabilities of moving from being a party that is confined to protest to being a party of – shared – power.

To have any significant political future, Liberal Democrats must be able to believe that they have made a Conservative-led Government behave differently. And while some might argue that they have helped Mr Cameron more than they have constrained him –by giving him political cover for policies unpopular with his Eurosceptic right wing – Mr Clegg can justifiably claim that the Liberal Democrats will not come away from their spell in government empty-handed. Whether he can convince all his party faithful of this, let alone wavering voters, however, is another matter.

There will be those – the party's president, Tim Farron, apparently among them – who feel that he could have pushed harder on tuition fees. But on income tax thresholds, on schools (the pupil premium) and on the green agenda, the Liberal Democrats have the beginnings of a record to defend. In his speech yesterday, Mr Clegg indicated lines that he would not be prepared to cross – tax fairness and green energy among them.

But the closer the election comes, the more difficult the Liberal Democrats' task becomes. The more they will have to differentiate themselves in policy terms from the Conservatives and the more they will need tangible achievements to show. Yesterday, Mr Clegg steered skilfully enough between the Scylla of social protest and the Charybdis of Conservative power. But he should be under no illusion; equally treacherous waters lie ahead.