The Greens' five-cylinder car

Motoring enthusiasts will remember the promises made when manufacturers introduced the five cylinder car.

It would have the economy of a four and the power of a six, they promised. Buyers discovered the power of a four and the thirst of a six, which is why we're not all driving five cylinder cars.

With their attention to economical motoring and aversion to power, the Greens on the weekend announced a political five cylinder car: They wouldn't rule out joining a National-led government, but the combination would be "highly unlikely". (Notice how the emphasiser somehow weakens the unlikeliness?)

Their aim is to reassure two opposite groups of voters:

- Anti-National Green voters must understand their vote for the Greens will not help Gerry Brownlee start open cast mining in National parks, or help Paula Bennet throw a few solo mums to snarling Rottweilers.

- National voters who are interested in recycling can be assured that Greens are not anti-National at all, as long as National stops butchering sea lions, decriminalises weed and takes strong action against sugary breakfast cereals.

The Greens are trying to have it both ways, and in doing so they risk having neither.

If I were a Labour candidate hunting Greens, I would get up on the hustings and say 'I agree with the Greens that we need action on climate change, but the Greens are refusing to rule out supporting a National Government.'

If someone wants to be sure of changing the Government they have one less reason to vote Green, even though the Greens' stance is targeting the 'change' vote.

And if I were a National candidate I would still try to marginalise the Greens with lime-tinged flatulence: "The environment is a mainstream concern for us all, but the Greens are not mainstream."

The Greens' problem is twice-over of their own making.

They have tried to resolve a hard choice by pretending no hard choice needs to be made.

They have a difficult and crucial strategic choice - either they can support a National Government or they can't. Either option carries risks of losing votes, and of reassuring voters. But you have to make a choice, not close your eyes and pretend it doesn't exist.

Their other self-confected problem is talking government-formation tactics at all. The Greens successfully, and a little cynically, constructed an appealing ''beyond politics" identity, trying to stay above the messy and self-serving politics of deals and horse trading.

Their government-formation options are an opportunity to highlight their reason for exiting. They should say, "We will work however we can to prevent climate change and ban cars. We will support the government that best promotes these crucial outcomes."

Then let the consequences fall where they will, with the other parties collecting all the backlash of playing politics.

That would take a hard decision: More power? More economical motoring? They need to decide if they want four cylinders or six.