Leading article: The third party has a chance to come into its own

Liberal Democrats can profit from the disenchantment of many voters

This is almost certainly the last weekend before the general election is called. But that does not make it a politics-free zone. On the contrary, the past week has seen the political temperature rise several degrees, and some tantalising shifts have been observed. Like it or not, the campaign proper has finally begun.

In sum, this was the week when the Conservatives held steady, Labour's standing declined and the Liberal Democrats made gains. Yesterday's ICM poll showed Nick Clegg's party running only six points behind Labour, while a ComRes poll for The Independent at the start of the week showed the Liberal Democrats clearly benefiting from disenchantment with both major parties and more than one-third of voters positively in favour of a hung parliament.

What cannot be known, of course, is whether this week's shifts represent trends that will strengthen as the campaign goes on or whether they will turn out to be just another set of twists in the mood of an electorate that will turn out to be as fleeting as the last. Will the gap between the Conservatives and Labour continue to widen, and if it does, will that reflect a strengthening of the Conservatives or a weakening of Labour? Will the Liberal Democrats be able to keep up the momentum that seems to be theirs?

Labour, it hardly needs to be restated, had a bad week. Any bounce from Alistair Darling's "steady as she goes" Budget had dissipated, and Gordon Brown, while making a well-judged attempt to broach the vexed issue of immigration, came a cropper within hours, when the Office for National Statistics discredited some of his figures. Coming so soon after Mr Brown himself had had to correct evidence he had given to the Chilcot inquiry on military spending, this left a sour impression of "spin", even if both were honest mistakes. Something is wrong if a Prime Minister and party leader facing an election does not have the resources to get his figures straight.

In fact, Labour was doubly penalised. For while Mr Darling more than held his own on the televised Chancellors' debate, his efforts were largely negated when business leaders publicly backed his Conservative opposite number on National Insurance rates. Not for the first time, George Osborne's political instincts had given him a winner. The initial response to his announcement had been that it was poorly timed for making headlines (on the morning of the Chancellors' debate) and unaffordable, if it was supposed to be funded by nebulous "efficiency savings".

The enthusiastic response from business leaders, however, turned that around. The question was no longer "How are the Tories going to afford that?" but "Is this the end of the love affair between big business and New Labour?" which was one of the keys to Tony Blair's 1997 victory. It also kept National Insurance in the news, alerting ordinary voters of a pending tax rise that might otherwise have passed them by.

But it is the Liberal Democrats who are entitled to feel most cheerful as the official campaign opens. Vince Cable confirmed his rapport with the voters at the Chancellors' debate, while his lower national profile recently has allowed Mr Clegg to step into the limelight. After last week it is also clear that being the third party has benefits in a three-way discussion, and the first leaders' televised debates give the Liberal Democrats their first real chance to reach a wider audience on equal terms with the other two.

Mr Clegg is emerging as an able and increasingly confident advocate before an electorate that has had enough of Labour, but remains as yet unconvinced by the Conservatives. The Liberal Democrats are not helped by our electoral system, but it is a compliment to Mr Clegg that they enter the campaign proper as serious players and with a sense that 2010 could be their year.