KEY POINTS:
The Greens face an extremely tough decision on Tuesday - one likely to have a huge bearing on how the party fares at the upcoming election.
On the face of it, it would seem unlikely the Greens will eventually vote against Labour's emissions trading scheme. If nothing else, it is surely better to have something in place than nothing at all.
And if the polls ring true and National leads the next Government, it is hard to imagine the Greens being happier with what John Key draws up.
But it is clear that the Greens are struggling with the decision about which way to vote.
And this is not surprising.
Co-leader Jeanette Fitzsimons has for years been trying to get local politicians to recognise the issue of climate change and to put a price on carbon. She probably understands the issue better than any other politician in this country.
As much as she might want to have an emissions trading scheme, Fitzsimons also wants one that she knows will be effective and fair.
The problem is the Greens think it is unfair to have the agriculture and transport sectors left out of the scheme for some time yet.
The party is worried that too much of the costs of the trading scheme will be lumped on to people other than the actual emitters.
Fortunately, the Greens' concerns that the scheme simply was not going to reduce emissions appear to have been partly soothed by changes made during intense negotiations.
If they back the legislation they will face some difficult questions from their core constituency. They might be seen to be acting as Labour's "handbag".
But if they vote against, they'd better be ready for an icy blast from a wounded Labour, which is desperate to pass its flagship bill.
Such a situation might make the public wonder, come election time, whether a centre-left Government involving the Greens is really a viable option to lead the country.
The stakes are high, and the Green MPs know it.
