What looked like a campaign set to be dominated by third parties now has suddenly been tilted back towards the big two. Jacinda Ardern's election to the Labour leadership makes many new things possible, but one key thing even more likely

Beware cries of a Labour miracle. While Jacinda Ardern is "a young proposition", she's not just been pulled from the bullrushes, and while the past 36 hours have seen a remarkable 'Jacinda Effect', she's not the saviour. But she has changed this election campaign utterly.

The election of Ardern to the Labour leadership, after Andrew Little signed his own execution warrant by admitting he was thinking about wielding the axe himself, has sparked a remarkable reaction; one we haven't seen for many a year. Perhaps it's partly reporters' relief that they will now not have to endure seven weeks of electioneering by two strikinginly uncharismatic men. Or, to be fair, perhaps it's the logical calculation that Labour was starting to flatline, but now Ardern's 'game-changer' qualities kick-started the party's heart and the contest is on again. That's certainly the message anyone keeping even half an eye on the headlines this week will have taken away.

So exactly how has the game changed? Overall, what was looking like a third party election, now potentially tilts back to the big two.

But let's look at it one party at a time, starting with Labour itself.

While a week ago I might have said the party was battling away – and certainly the people I spoke to were talking a good fight – the sheer beaming enthusiaism of the past day and a half shows just how deep in the shade it was. It's night and day. The base is energised and – with reports of 1000 new volunteers in little over a day – expanding. 

Now, for a short while, voters will take another look at Labour. She has the chance to contrast herself with both the percieved incompetence of the Labour Party of recent years and the 'stale, pale and male' National Party. Hence her promise of "relentless positivity".

At a notable first press conference, Ardern showed the past three Labour leaders how to make a first impression, something they all failed to do. She was strong and decisive. Then, to use a sporting metaphor, the pressure she had created paid off, when she created a 'moment'. Turning to Mark Richardson on the AM Show, finger pointing, and speaking on behalf – not of herself – but of all women, she showed her fighting spirit and, more importantly, that being young does not mean she's timid. Ardern told Richardson he had no right to expect that women should discuss their fertility plans with employers. A lack of surety has dogged Labour for years, but that first impression showed confidence and that most precious thing, authenticity. Voters will note that.

Perhaps, most of all, women voters. And that matters. One of the most important things John Key did was lure women away from Helen Clark's Labour government. Ardern will need to win them back and this gave her an opportunity to take a stand that many women will appreciate.

Yet it will tend to be appreciated more by women – and voters in general – who are already in tune with her values. So it's not an issue Ardern wants to dwell on. There's little point in her simply winning the liberal vote back off the Greens. She needs to tack to the centre, a lesson surely drummed into her during her years working for Clark. She needs to take votes off National and New Zealand First if she wants a Labour-led government.

For all the initial hype, that won't be easy. Little wasn't helping Labour, but he can hardly be blamed for voters' lack of belief. The polls slumped to record lows after the interns fiasco and Metiria Turei's passionate benefit fraud admission, which simply reinforced perceptions that Labour lacks competence and something to believe in.

'The Ardern Effect' has to overcome a strong economy and stable government. Labour's baggage might be hidden behind Ardern's radiance for a few days, but it's not disappeared. Labour isn't suddenly no longer a bit of a mess. The change of leadership has made some things possible again, but it's far from a slam dunk. The next week and whatever new policy Ardern announces to make her mark is vital. It must appeal to the centre, not the left, of her party. And she has to learn how to ignore endless advice and continue to be herself.

Talking about centre voters... National. In some ways this changes little for Joyce and the strategists. The central message that it is a safe pair of hands and a Labour-led government could lead to chaos is potentially more potent with an untried 37 year-old at the helm. National will still spend much of the campaign crying "unstable, unstable". 

However with just seven weeks to play, however apt that message, it may struggle to get cut-through if Ardern's relentless positivity soaks up the attention. 'Strong and stable' may suddenly be the worst kind of message. But it's who Bill English is, so they will struggle to pivot from that. National risks looking complacent and stodgy in contrast to the fun, fresh party to its left. Boredom is now its greatest threat and so this potentially changes everything. Confused yet?

National will need to create some jeopardy around Ardern. Her age offers some potential, especially in contrast to a potential coalition partner in Winston Peters. But it's treacherous ground for older men. It's not sexist to doubt the wisdom of electing someone who has questioned her own readiness to lead the country and would be New Zealand's youngest ever Prime Minister. But you can bet, just as National did she she was elected deputy, (calling her "a superficial, cosmetic facelift" for the party) it will be women ministers such as Paula Bennett, Maggie Barry and Nikki Kaye who will be the most scathing. 

It's hard to avoid the conclusion that National aren't at least a little scared. It's been proven in the past that governments who deliver sound economies can rue the day, when that very economic security emboldens voters to take a punt on something new.

But National still has competence on its side. It has hip-pocket trust. It has the default vote of many. The party is united, familiar and there's no great groundswell for change. What's more, it has money to spend on new promises if it decides a bribe is in order. 

The Greens have been rewarded for doing their job by finally prodding the sleeping giant into action. It may yet be too late, but Green MPs would say they met their part of the bargain implicit in the Memorandum of Understanding by getting to 15 percent. Labour had failed to hold up its end by not getting to 35 per cent. The problem with that line is that it's been a zero-sum game between the two for nigh on nine years, as they essentially took vote from each other. Both have failed to grow the pie.

Although James Shaw is already using the line that 'a party vote for the Greens is a vote for Ardern as PM', she could just steal back the votes Turei has so cleverly won. Now they need to convince Labour to let them do their work on the left, while it looks to plunder National and New Zealand First. But Labour MPs are hardly loving the Greens at the moment, blaming them in part for the poll slide that started all this. So it'll be interesting to see how those tensions are managed now that both parties have taken the gloves off and are going full tilt to maximise their own party vote. 

The Kelvin Davis Effect creates all sorts of problems for the Maori Party. Maori voters who felt Labour was not showing them enough respect are suddenly confronted with a culturally connected Maori on the party's billboards. That should staunch vote loss to Flavell and Fox. But more importantly it may make it a lot harder for Howie Tamati and Hone Harawira. If it's possible, Waiariki becomes even more vital, or the Maori Party could be toast.

But, as so often is the case, Winston Peters and New Zealand First are a big winner from the politics of the past couple of days. Oh, that vain hope that New Zealand First might almost catch Labour and Peters might be in a position to at least share the Prime Ministership may well be gone. A sustained burst of enthusiasm for Ardern would stop Labour sliding into 'end of an era' territory. But that was always a looong shot.

More realistically, Ardern gives Labour the chance to climb back to a number that begins with a three. That means less momentum and attention for New Zealand First, but more importantly it all-but guarantees Peters' kingmaker status. While Labour's continued slide may have meant more vote for New Zealand First, it may not have made the party more powerful. It could have meant National could have carried on with its existing partners or simply meant that Labour and the Greens were so far behind National that New Zealand First had no negotiating power and no choice but to prop up a National-led government.

A stronger Labour Parthy – if that in fact ensues – actually means a more powerful Peters. He gets more leverage, more ability to play the two parties off against each other, and more real choice. 

So in a funny way, the Jacinda Effect changes very little and simply makes what has looked like the most likely scenario for the past year – Peters as kingmaker – even more likely. On the other hand, it changes everything and the sense of uncertainty and risk is higher than ever. MMP eh? Sometimes it doesn't make much sense. But it makes for a fascinating two months.

 

Tune into hear me discuss all these issues with Guyon Espiner and Lisa Owen on RNZ's politics podcast Caucus. It's out every Thursday and you can subscribe on iTunes or listen at RNZ.