Caroline Lucas: The Lib Dems' green policies are wilting

Cross-dressing seems to be this season's political fashion. While the Tories are courting the Lib Dems (with "scarcely a cigarette paper" to slip between them, according to David Cameron), the Lib Dems are showing a bit of ankle to potential Green voters. But whereas Nick Clegg's recent overtures in this direction might reflect the growing threat we pose to his party, they don't reflect any real meeting of minds in terms of a common agenda.

In their recent rhetoric, the Lib Dems appear to want to emulate the Green Party. And over the years their official policy has clearly followed that of the Greens in a number of respects. But the Lib Dems' practical track record is something else altogether. Historically, this is a party that has been characterised by what you might call "consistent inconsistency" – saying one thing and doing another, and following different policies in different places regardless of their supposed principles.

Take roadbuilding as an example. Back in the early Nineties, the Lib Dems called for a moratorium on new roads, and today Norman Baker is saying that a Lib Dem government would halt spending on roadbuilding. The rhetoric may have been consistently green but the practice, with equal consistency, hasn't been. From the Newbury bypass and the Batheaston bypass, right up to the M74 extension and the Lancaster northern bypass, you'd be hard pressed to find a major road scheme in the past 20 years that the local Lib Dem MP or councillors haven't supported.

With aviation, the Lib Dems are inconsistent in a different way. They opposed the expansion of Heathrow, but have been happy to support the expansion of Birmingham, Carlisle, Exeter, Liverpool and Norwich airports. They were wildly enthusiastic about Manchester airport's second runway – except for Lib Dems in Stockport, under the flightpath.

Greens would say that air pollution knows no boundaries, but for the Lib Dems it depends where you are. In the 2002 local council elections they gained control of Hull by campaigning against an incinerator, and on the same day lost control of Sheffield where they had called for a new incinerator. They currently support incinerator projects in Exeter, Plymouth and Barnstaple. In Essex they support a zero waste strategy, which ought to mean no incineration – but yes, Essex Lib Dems are still supporting incineration.

As most people now know, being Green isn't just about the environment – it's about social and economic justice too. Clegg's recent lurch to the right, his call for "savage cuts" in public spending, and his pronouncements on tuition fees and child benefits will therefore do little to endear him to Green voters. In fact it's unlikely to endear him to that huge constituency of voters in the centre and on the left, either.

Those people, millions of them, are looking for something that isn't Labour or Conservative, that doesn't take its orders from the finance sector. A party with a genuine commitment to fairness, which promotes tax increases for the very wealthy because it's just. A party which wants an urgent crackdown on tax havens, bonuses and chief executive pay, instead of offering to cut public services – just because that's what Labour and the Conservatives want too.

The Liberal Democrats have always been a party with an identity crisis. In part, this has been fuelled by uncertainty as to whether they should bid for the disillusioned Labour vote or the disenchanted Conservative vote, knowing always that none of these voters are heart-and-soul Lib Dems. And part of the identity crisis is that the Lib Dems dearly want to be the Greens, but they can never quite manage it.

Caroline Lucas is leader of the Green Party of England and Wales