Donald Macintyre: The Conservatives' dilemma is even worse than Labour's

Even if David Cameron were defeated on 6 May, he would have earned his place in political history by causing arguably the greatest change to British electioneering in a generation or more. When I asked him on his campaign bus on Friday how worried he was by the positive impact for the Liberal Democrats of last Thursday's television debate, he showed every outward appearance of being philosophical, pointing out – with justice – that people would now wonder why such debates hadn't happened for the past 20 years.

Michael Cockerell's riveting BBC documentary about TV debates – their history in the US and their non-history in the UK – is a timely reminder of how successive British front-runners (Thatcher and Blair included; the latter amid considerable obfuscation) had rejected the idea. It is easy for now to gloat at the consequent Liberal Democrat surge, most of it so far at the Tories' expense, but the debates are surely here to stay. And whatever his reasons, by taking the risk Mr Cameron has reignited interest in electoral politics at a time when it was at one of its lowest ebbs.

Whatever the longer-term impact, however, the immediate effect has not been good for the Tories. The sudden centrality of the debates may offset some of the financial imbalances between the parties, one of which Labour, in particular, has been complaining about. The glossy leaflets and hugely expensive posters churned out in marginals with Lord Ashcroft's cash may not mean that much (assuming they ever did) if up to 10 million electors choose to see the party leaders in their own living rooms. Clegg has become the story of an election that badly needed one. But yet no one knows for sure what the Clegg surge, especially as shown in the most dramatic of yesterday's polls, means.

On the most minimalist reading, it represents little more than what one Labour strategist yesterday described as a "holiday romance" – albeit a strikingly passionate one – that the electorate will start to put behind it as the campaign wears on. On another, it presages the first return to real three-party politics since the 1980s, and perhaps a new era in which neither of the biggest two parties can govern without the support of the third.

Either way, however, it poses the other main parties with some obvious dilemmas, the most acute of which is for the Tories. David Cameron will surely hesitate to launch the traditional, full-frontal, right-wing assault on the Liberal Democrats for being "soft" on crime and immigration (or even defence and Europe) which risks alienating exactly those younger, socially liberal middle-class voters he has made a good job of courting in the past two years. Although he has criticised those policies on the stump and, presumably, will continue to do so, his preferred central message yesterday was that the "real" contest was between the Tories and Labour. If the poll results are sustained, he may find it more difficult to resist the internal pressure to go more fully on the offensive.

But there may also be subtler dilemmas for Labour. If even part of the Lib Dem poll surge was translated into reality, Labour has seats to lose too, which is no doubt why Gordon Brown, in his interview with Andrew Marr yesterday, began to attack Lib Dem policies such as restricting tax credits and ending child trust fund payments. Nevertheless, Mr Brown, like his colleagues, knows Labour's only real hope of staying in power (one that has broadly been boosted by the Liberal Democrat surge) rests with a hung parliament, and a possible coalition that would actually provide the stability necessary for the extremely tough economic times ahead.

The arithmetical range within which that becomes a realistic possibility is relatively narrow: Labour and the Liberal Democrats would have to be able to form an overall majority on their own, with the Tories not too far ahead. And even then the Liberal Democrats may well decide to steer clear of joining a government, particularly one led by Mr Brown. The Prime Minister was surely right yesterday to insist that he was seeking an overall majority. In the last week of the campaign, however, he may have to decide whether to follow in the footsteps of his Transport Minister, Lord Adonis, and go out of his way to encourage tactical voting by Liberal Democrat electors (and, by unspoken implication, Labour voters) to keep the Conservatives out. It isn't impossible that he will. His speech on constitutional reform at the beginning of the campaign helped to prepare the ground, if he chooses to occupy it.

The danger, of course, is that this would simply look like an act of desperation, as Neil Kinnock's 11th-hour overtures to the third party did in 1992. To prevent that, Mr Brown needs to make his move from a position of relative strength. Focus groups questioned about Thursday's debate apparently showed no sign of warming personally to the Prime Minister. But when asked if they agreed with him, especially on the economy, they are said to have been notably less anatagonistic. Mr Brown's greatest danger is to be seen as a man of the past squeezed out of a Clegg-Cameron debate about their visions of the future, however cloudy they remain when it comes to eradicating Britain's huge structural deficit. Mr Brown's main vulnerability is the question repeatedly posed during last Thursday's debate, of why he hadn't done in the past 13 years what he wants to do now.

Not all of this is personally fair. On constitutional reform, for example, it was John Prescott rather than Mr Brown who set his face against a deal (embracing electoral reform) with the Liberal Democrats, which may account for the Prime Minister's suggestion to Andrew Marr yesterday that the "history books" would show he was not opposed to such an accord. And it was Tony Blair himself who effectively abandoned serious reform of the House of Lords.

Neverthless, Mr Brown may paradoxically increase his credibility for the future by admitting more readily the errors – his, and those of others – of the past. Last week, Mr Brown did finally acknowledge that the Government was wrong to fail to regulate financial services more actively when it was first in power. He was by all accounts persuaded to do so – not least by his Chancellor, Alistair Darling – on the grounds that people would actually respect him for it. Yet his reformulation of this point in an otherwise assured performance on The Andrew Marr Show yesterday was again somewhat grudging and partial.

Mr Brown suggested that Labour would benefit when the debate started to focus on the economy including, by implication, his widely praised handling of the credit crunch. And even if it would require heroic determination by a Labour-led government to carry it out, the party has at least (in the Budget Red Book, if not in the manifesto) outlined its approach to the deficit. It is by no means impossible that a sizeable portion of the currently floating electorate will finally decide that Labour has the judgement and experience to lead Britain out of the recession, and without decimating public services, and even more so if there was a chance of a coalition with a resurgent Liberal Democrats.

Yet without renouncing the past Mr Brown needs to be more explicit in making it clear that his party has a new approach for a new era. And that may require admitting Labour's past mistakes as well as its achievements.

d.macintyre@independent.co.uk