I am not a voter of the left, but if I was I would struggle to find a reason why I would not currently support the Greens over Labour.
Before I explore why the Greens are doing so relatively well at the moment, first let us look at perhaps the historical reasons why left voters generally did not support the Greens.
The first is policy influence. The two larger parties are the one that can shape Government the most. A party on five per cent has little influence compared to a party on 45 per cent. But this is not the environment in 2012. Labour continues to poll in the 20s only, and the Greens have started to poll between 14 per cent and 17 per cent. This means that in any future Government, the Greens could represent a third or more of the Government, which would give them massive influence - at a minimum Russel Norman could expect to be made Minister of Finance. If the Greens grew five per cent further, then they could lay claim to top job. I am not saying they will achieve this, but I am saying that in any future left Government, the Greens are looking likely to have great influence. There is no reason to think that their future role has to be that of an minor support party.
The second reason why the Greens historically did not attract widespread support from the left is because they were perceived as extremists. Their defence spokesperson hated the United States. Their justice spokesperson was best known for breaking the law. Their economic policy was to argue against economic growth. Their consumer spokesperson railed against the size of easter eggs, and their welfare spokesperson was a former beneficiary rights activist. At one stage a significant portion of their caucus were actually former Marxist and maoists.
Now the Greens have managed to be perceived as far less extreme. They talk about a richer New Zealand. Their transport spokesperson doesn't rail against the evils of motorcars, but instead can debate cost benefit ratios and hidden subsidies with ease. Their health spokesperson is a former DHB chief executive. The Greens' language and rhetoric is far more mainstream than a decade ago (even if the policies themselves have not changed a lot).
So if one puts aside the historical reasons why a left voter didn't support the Greens, what are the factors which would mean a traditional Labour voter would remain with Labour? Is it they prefer Labour's policies?
Well if you look at almost all the new policies adopted by Labour in the last couple of years, they stole them from the Greens. Capital Gains Tax - was Greens policy. Extending the in work tax credit to beneficiaries - was Greens policy. Modifying the Reserve Bank Act targets - again Greens policy. Paid Parental Leave extension - forced on Labour by the Alliance, and pushed by the Greens. A hike in the minimum wage to $12 and then $15 - Greens and NZ First policy for many years. A tax-free threshold for all earners - again long-standing Greeen policy.
So the policy differences between Labour and the Greens are shrinking rapidly. How many Labour voters can name a Green policy that they actually disagree with, and cite that as why they do not support the Greens? I suspect it is more tradition, than policy.
There may be reasons other than policy, that people support a party. It could be how united a party is. The Greens have never had a leadership coup, or even leadership speculation, while Labour is rife with tension. The Greens were widely praised for their list ranking and new caucus talent, and Labour heavily criticised for protecting non-performing incumbent MPs.
If industrial relations is what matters to you, then the Greens are definitely pro-union, but are actually independent from them.
If economic issues are to the fore, then you may have noticed that Green Finance Spokesperson Russel Norman has put out twice as many releases as Labour Finance Spokesperson David Parker, and been in around five times as many news stories. Many journalists have commented that Dr Norman has become the de facto shadow Finance Minister.
If you don't like MPs spending their life in Parliament, you might note that none of the Green MPs have served more than 10 years in Parliament. All of their original 1999 MPs have retired. By contrast Labour has four MPs who entered in the 1980s, and a further seven who entered in the 1990s, and failed to rejuvenate their List MPs in 2011.
Most of all, the Greens have a clear brand, clear principles, clear values and a united caucus. Labour is wallowing in trying to reinvent itself as standing for something beyond being "anti-National".
So a genuine question to readers who tend to vote left. If you still support Labour over the Greens, why? From a rational analysis point of view, the Greens seem to be the superior choice for a voter of the left.