TV3 has a new poll out tonight. Simon Wilson unpicks what it will mean for the different parties if that poll reinforces the trend to Labour revealed in TVNZ’s poll three days ago.

Warning: this story contains some outrageously unprovable assumptions.

Here’s an interesting proposition about the election outcome, based largely on one big assumption: that National’s John Key voters – those people who switched to National because they believed in John Key – are about to flick over to Jacinda Ardern.

The proposition is this:

National: 35%, which turns into 43 seats.

Labour : 45%, 55 seats.

NZ First: 9%, 11 seats.

Greens: 6%, 7 seats.

Māori: 1%, 2 seats.

Act: 0%, 1 seat.

Others: 4%, 0 seats.

With these numbers Labour will be able to form a government with support from either the Greens or NZ First, but it won’t need both. The details are explained below. They are also the numbers that show how a surge to Labour at National’s expense will mean National cannot form a government.

We’ve already seen the surge to Labour. We have not yet seen a surge away from National. Tonight’s poll will reveal if it’s started.

While this proposition is based on the one big assumption – essentially, that Jacinda is the new JK – there are many other little assumptions backing it up. As follows …

1 The Greens vote

When the Greens crashed from 15% to 4%, they did it by shedding votes to Labour, National, TOP and even NZ First. But then some of those former Greens supporters realised they still want the party in parliament, so they drifted back. This has destroyed TOP but has not hurt the others much, if at all.

The Greens have now adopted a steady, environment-focused approach to the campaign, aiming to reinforce their role as the true green party. That should get them to 6 or 7%. If they get a roll on in the next two weeks, which is possible now Labour has stopped trying to eat their votes, they could get a little more. It wouldn’t hurt if James Shaw showed us a little more of those moments when he can be relentless positive.

2 The NZ First vote

NZ First always polls better on the day than it rates in opinion polls during the campaign. On that basis, you can expect it to do better than the 10% it currently has in the poll of polls.

But. It has almost never slipped in the polls during a campaign, as it did in the Colmar Brunton poll last Thursday. And that slip was a quite large 20% of its vote. On that basis, you might expect it to keep slipping.

Many NZ First supporters were Labour people running from Andrew Little (and his predecessors), but Jacinda Ardern has won a lot of them back. She’ll get many more before she’s finished, because of the Jacinda effect. Everyone loves a winner.

National will get NZ First voters too, because actually not everyone loves a winner: some people really don’t want an urban liberal Labour government and/or a young woman in charge. But there will also be National voters who decide there’s no point in sticking with the National losers and jump to NZ First, to strengthen it as a coalition partner for Labour.

My guess: probably more will go from National to NZ First than the other way around. But the biggest trend will be from NZ First to Labour.

3 The Māori vote

The Jacinda effect will probably help Labour to a big sweep in the Māori seats for the party vote. Even the Greens’ Marama Davidson in Tāmaki Makaurau and Metiria Turei in Te Tai Tonga will be powerless against it. And in the electorate votes, it could also mean a clean sweep.

But if the Māori vote splits, party and electorate, the Māori Party could do okay with the electorate votes. Te Ururoa Flavell is still favoured to hold Waiariki; Howie Tamati is rated with a good chance in Te Tai Hauāuru and Shane Taurima has a chance in Tāmaki Makaurau too. The others do not seem to be play. Up north, the Mana Party’s Hone Harawira in Te Tai Tokerau is unlikely to resist the Jacinda/Kelvin Davis onslaught.

So the Māori Party could win three electorate seats. That would mean co-leader Marama Fox misses out. But if the Māori Party wins only one seat, and gets over about 1.3% of the total party vote, it will be allocated a second seat off its list, and that will go to Fox.

4 The ACT vote

National has destroyed the ACT Party, while keeping it alive with a single seat in Epsom. If you thought no one was listening to Andrew Little, spare a thought for the even more ignored David Seymour: once acclaimed in the press gallery as one of the stars of the government’s current term in office, he is now utterly irrelevant, in electoral terms, to everyone except those odd Epsomites.

5 The National and Labour votes

National has lost a little to NZ First and Labour, but right now it’s standing on the edge of a precipice. And it looks like Labour might just push it off.

Labour rose in the polls on the backs of the Greens, with Jacinda Ardern successfully positioning it as the environmental party, the climate change party and the anti-poverty party. Since then, though, Ardern has pivoted and focused more on the party’s centrist positions (on tax, immigration, water pollution and more). Expect to see more of that. She’s going after the NZ First vote now. And much more importantly, she’s going after National.

National’s buoyancy has been John Key buoyancy. They’ve always known it and for nine straight years (including election year 2008) the magic of John Key was pretty much all they ever talked about. It stayed in place after he left because 1) people weren’t too unhappy; 2) the Key government was still there and Bill English seemed nice enough; and 3) nobody liked Andrew Little.

Now 3) has gone and Labour has found its own super-charisma machine. That, in turn, has made 1) and 2) irrelevant. Everything is about to change.

The single question dominating this election is now: Would I like Jacinda Ardern to be prime minister?

And on the strength of her campaign performance to date, who, really, apart from core National supporters, is going to answer no? How big is that core? We know from the last election when Bill English was leader that the answer is: only about half what they’re polling now.

Jacindamania has achieved the remarkable feat of taking Labour from 23% to 43%, in just one month. But it did that largely by sucking up all the votes it could on the centre-left. The next stage is to suck whatever votes it can find on the centre-right, and that’s barely started. Jacinda Ardern has John Key’s remarkable ability to communicate directly with voters. As National’s John Key supporters experience that, and see that she’s a winner, they will flock to her.

Remember how, for nine years, centre-left strategists tried every way they could to undermine John Key? They could just never do it. Despite their best efforts, despite Key’s own dithering leadership, the scandals of his government, the growing howls of outrage from everyone who felt they were missing out. None of it mattered. His appeal never wavered and, because of that, nor did his party’s. These things may not be beyond reason, but they certainly defy conventional explanation.

Now it’s Ardern’s turn. The factors are different – the appeal of a confident young woman is vastly different from that of a confident super-wealthy middle-aged man – but the way it works is fundamentally the same. The appeal is subliminal and, for an awful lot of people, irresistible.

The lesson is that right now there’s not much the National Party can do about it.

The big day: options for the next government

So assuming tonight’s poll doesn’t throw everything to the wind, what does Labour need from the election to form a government? And what does National need to stop it? Here are those plucked-from-the-sky numbers again.

In a 120-seat parliament, Labour can form a government with NZ First or the Greens. With NZ First it will have 67 seats, a very comfortable majority. With the Greens it will have 63 seats. That’s more than National has now, and easily enough to run a government.

A few words on the calculations. The numbers are rounded, obviously. They are also “adjusted” (from the first column to the second) because after the election, all the votes for parties not in Parliament are discarded. In this outcome, that’s 4% (TOP and other). All the other parties’ results, which total 96%, are then adjusted upwards, to bring the total back to 100%.

This means that winning 45% in the election, in this outcome, effectively means winning 47%.

I’ve assumed the Māori Party will win two seats, despite winning only 1% of the party vote. I’ve assumed Act will win Epsom, despite winning 0% (okay, rounded down) of the party vote. That could create an overhang, with more than 120 seats, depending on the precise distribution of the votes. But these calculations don’t produce an overhang: we’d have a parliament of 120 seats.

What if it’s not quite like that?

Indeed. What about the Māori Party, which wants to be in government whichever party does best? If Labour loses 3% off the (adjusted) total shown, winning only 44%, and the Māori Party picks up 2 of that 3%, that would bring the chance of a Labour/Greens/Māori coalition to the fore.

Labour would have 53 seats, the Greens 7 and Māori 4. Total: 64. But Labour could still go with NZ First, ignoring both those other parties. Total: also 64.

If Labour does worse, if the Greens do worse, if NZ First does better, the calculations change. The Greens, in particular, will regards 6% as being far too close to the 5% cutoff for being in parliament at all. They hope to do much better.

What’s the bottom line for Labour and the Greens to form a government? 49%.

If the discarded vote is low (4% or less), the combined Labour/Greens vote on the night will need to be 49% or better. But they could still do it with less. The opposition would comprise National, NZ First, Māori and Act, and there’s no way that lot will be able to form a government together.

For a Labour-led government the magic figure is 49%. If they can get there with the support of the Māori Party as well as the Greens, they will form the next government.

And National?

Assuming the same low discard vote (4%), National will be able to form a government if it can command that same 49%. It’s not likely to do it on its own. Or just with ACT. But if the Māori Party wins 3% and ACT wins 2% and National manages to hold 44%, that would be a government.

For National as for Labour, things are much simpler if it does not have to turn to NZ First, because NZ First doesn’t want to work with the ACT, Māori or Green parties.

But perhaps National and NZ First could win 49% between them. National on 37% and NZ First on 12% would do it. National on 35% and NZ First on 14% would do it too.

But really? If National slips into the 30s and NZ First goes into the teens, a government they formed would mean NZ First was propping up a National Party that had effectively lost the authority to rule.

NZ First would have won those extra votes from Labour, but Labour would still probably have 40% or more of the vote. Under this outcome, Labour probably couldn’t form a government with the Greens but it would easily have the numbers to do it with NZ First, and NZ First could well find the logic impossible to resist.

All of which means that at this point, if the trends continue, National will find it very hard to form the next government at all.

So, will it be the Greens or NZ First?

The question is, will Labour be able to do it with the Greens, or will they need to turn to NZ First? Or will they attempt to do what National has done this term, turning to one support partner or the other depending on the bill in question? That would be possible too.

Because despite the memorandum of understanding, do we really know if Labour has a preferred partner? More on that, very soon.

simon@thespinoff.co.nz @simonbwilson