Leading article: So Mr Clegg's adventure continues

If either Labour or the Conservatives were hoping that the second party leaders' debate in Bristol last night would see the Liberal Democrat genie forced back into the bottle, it was soon obvious they were destined to be disappointed. Both Gordon Brown and David Cameron altered their approach after the extraordinary success of Nick Clegg in last week's groundbreaking debate in Manchester. This time around, Mr Brown was keen to emphasise his experience. Throughout the debate, he cited past Labour achievements, from bringing in civil partnerships to reducing net immigration. And the Prime Minister was especially eager to move the debate on to the subject of the economy, which he, with some justification, still regards as his strongest card. There was also a marked attempt to differentiate Labour's platform with that of the Liberal Democrats. The phrase "I agree with Nick" was mercifully banished.

Mr Cameron said he was going to be himself in advance of the debate. He was certainly characteristically fluent. And he had made stylistic improvements, looking directly into the camera more often. But it was a strangely subdued performance. The Conservative leader only came to life at moments. He too sought to emphasise differences between the parties. But he lacked the conviction that many of his supporters were doubtless hoping for. As for Mr Clegg, he lived up to his performance of last week, managing to present himself (rather than Mr Cameron) as the true face of change in British politics.

So much for style. On substance, the debate was something of a let down. There was not the robust clash of ideas and policy that there could – and ought – to have been. On Afghanistan, the disagreement was about the tardy supply of body armour to troops, rather than the issue of whether British troops ought to be there in the first place. Mr Clegg chose not to emphasise his party's growing scepticism towards that campaign. This is a shame because the question of foreign military intervention is one of the great ignored fault lines of British politics. Labour and the Tories combined to support the invasion of Iraq and the Liberal Democrats, alone, opposed it. It would be a travesty if Mr Clegg acquiesced while these same two parties seek to shut down debate on our mission in Afghanistan in the same manner.

There was a question about the Pope's impending UK visit, but no one was going to offend Catholic voters by saying he should stay away, despite the growing doubts about the appropriateness of this trip. All three leaders scrambled for the votes of the elderly when a question was asked about pensions and ducked, once again, the crucial question of how society will fund long-term care.

On Europe things were more interesting. Mr Cameron seemed uncomfortable. He mouthed the old Tory Eurosceptic slogans about "taking back power" from an over-mighty Brussels, but with little relish. The best attack line on the Conservatives' isolationist policy on the EU came from Brown, who noted: "It's the Big Society at home, little Britain abroad". Yet Mr Brown himself had nothing hugely positive to say about our relationship with the continent, other than that noting the economic benefits. As the leader of a genuinely pro-European party, Mr Clegg had a credibility when he spoke about working with our European neighbours that the other two frankly lacked.

The other significant disagreement was on Trident, where Mr Brown and Mr Cameron ganged up on Mr Clegg. Yet the problem faced by both Labour and the Conservatives is that the Liberal Democrats are surely right to argue that it makes no sense to exclude the question of Britain's expensive nuclear weapons delivery system from next year's Strategic Defence Review.

Some have suggested that these debates amount to little more than "X Factor politics". That seems unfair. The indications are that these events are cutting through to the public. They seem to be engaging people who might not otherwise take an interest. That is something to cherish, not scorn. As for Mr Clegg, his remarkable adventure continues.