The Liberal Democrats are the wildcard of the forthcoming general election. Having improved their performance at the past two elections, it seems all but certain that they will now do so again. What is less certain is the impact this will have on the other two main parties and whom it will hurt most.

In both 1997 and 2001 the big losers were the Tories, with both Labour and the Liberal Democrats benefiting from large-scale tactical voting among one another’s supporters. Indeed, it is still the case that the overwhelming majority of Lib Dem target seats are currently held by the Conservatives – 24 out of the top 30, compared to only five for Labour.

But this time it could be Labour rather than the Tories who lose out most. Disillusionment is still running high among Labour’s traditional progressive voters as a result of the war in Iraq and other policy disputes, and the Liberal Democrats are likely to be the principal beneficiaries. Aware of this, the party is positioning itself to pick up as many disaffected Labour voters as possible – emphasising their opposition to the war, pressing for a new top rate of income tax and calling for tuition fees to be scrapped. Even if this does not result in the Lib Dems picking up many Labour seats, it could still harm Labour by eroding support enough to let the Tories make significant gains.

If Labour is to prevent this from happening, it is essential for the party to expose the fundamental dishonesty at the heart of Liberal Democrat policy-making. This is not going to be easy – as David Aaronovitch has argued, ‘there’s a special category of under-questioning that seems to apply to the Liberal Democrats, in that they are never, ever questioned about policies’. But we need to overcome this obstacle and show how, as a party of opposition, the Liberal Democrats feel no need to ensure that their policies are either consistent or deliverable. We need to show how they say different things to different people. And we need to show how, when the Lib Dems have achieved power in local government, they have failed to deliver on their promises.

At a national level, Liberal Democrat policy-making is opportunistic and unprincipled. The most outrageous recent example is on the issue of binge-drinking, where the Liberal Democrat shadow home secretary, Mark Oaten, has attempted to capitalise on increasing public disquiet over new pub opening hours by calling for the liberalisation of licensing laws to be delayed until the government gets binge drinking under control, particularly among the young.

This may seem odd to those who recall the then Liberal Democrat culture spokesman, Nick Harvey, enthusiastically supporting the licensing bill as it passed through parliament, and raising no concerns about the potential increase in binge drinking as a result of the reforms. Furthermore, observers might wonder how Oaten’s stance on binge drinking fits in with the recent call from Lib Dem culture spokesman Don Foster for the drinking age to be lowered to sixteen.

On transport policy, too, the Liberal Democrats have been opportunistic and inconsistent. Playing up their green credentials, the party has long supported the concept of congestion charging – even noting in their recent pre-manifesto that the idea was ‘first proposed by the Liberal Democrats’ and promising that ‘Liberal Democrats will extend congestion charging’. In practice, however, the party is much more reluctant to press for potentially unpopular road charging. In last year’s London mayoral election, Liberal Democrat candidate Simon Hughes adopted a distinctly lukewarm position on the issue, opposing any westward extension of the c-charge zone and proposing that the hours of operation for the charge be shortened to encourage people to drive into the central area in the evenings. Likewise, as transport spokesman, Don Foster was determined that the government should apply road charges in towns up and down the country, yet he opposed the idea in his Bath constituency. The party has also opposed the introduction of congestion charging in Edinburgh.

Such inconsistencies between national and local campaigning occur across a broad sweep of policy. For example, opposition to post office closures is a mainstay of local Lib Dem campaigns, with the party calling on the government to intervene to protect local services. Liberal Democrat leaflets rarely point out, however, that the party is committed to privatisation of the Royal Mail, where the party argues that ‘a privatised Post Office on Dutch lines could have a better chance of succeeding than the present structure’. Nor does that party point out that the Dutch post office has only half as many post office counters per head of population as its UK counterpart.

But it is at a local level that the gap between promise and reality is most striking, as shown in Newcastle.

When the Liberal Democrats captured Newcastle city council in June 2004, it was a worrying portent for Labour, showing that the Liberal Democrats could make inroads even in Labour heartlands. But the party’s performance since taking over the council has been a disaster, and provides several cautionary examples for progressive voters of the party’s inability to deliver.

For example, before taking over in Newcastle, the Liberal Democrats opposed the development of city academies, arguing that they would damage existing schools. Now, however, the party is proposing the closure of an existing comprehensive school in favour of a new city academy. Likewise, having said that they would invest in education, the ruling Liberal Democrats are now implementing cuts to special needs, harming some of the most vulnerable pupils.

And that is not all. Before being elected, the Newcastle Liberal Democrats claimed that they would protect the elderly and the vulnerable. Since then, however, they have imposed massive 28 per cent increases in social care charges, harming precisely the people they claimed they would protect. Furthermore, having said that they would spend public resources where they were needed most, they have done the exact opposite, channelling public funds towards middle-class areas at the expense of poorer neighbourhoods.

Finally, the Newcastle Lib Dems sought to establish their environmental credentials before they were elected by promising to end waste incineration. Since then, however, the ruling Liberal Democrat group has restarted incineration.

The behaviour of the Liberal Democrats in Newcastle is not isolated – it is emblematic of how they govern in other towns and cities that they control. Their u-turn on the incineration issue in Sheffield even prompted Charles Secrett, executive director of Friends of the Earth, to observe that ‘the choice between incinerators or recycling in Sheffield is a litmus test of the Liberal Democrats’ environmental and community commitments. It is the height of hypocrisy to have pro-recycling and anti-incineration policies, but then do the opposite in councils which they actually control.’

At the general election, more progressive voters than ever seem likely to place their faith in the Liberal Democrats. But with the Lib Dems escaping meaningful scrutiny, too few of them will ask themselves if the party is really all it seems.