If the National Party's worst case scenario happens and Jacinda Ardern becomes Prime Minister with Winston Peters as her deputy, the game-changing moment may be traced back to when John Key resigned at the height of his powers.

While he was probably right, for his own legacy, to leave at the height of his powers, it could prove disastrous for his party.

The key problem is that as much as National would like you to think that elections are all about having a stable economy, they are not - it is increasingly all about personality.

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And Sir John was probably the politician who brought about this change by making X-Factor a factor in who we choose to lead us. Also, when he left, he created a personality vacuum in the National Party and, more importantly, in New Zealand politics.

This created an environment for Mr Peters to thrive in his flamboyant way and also allowed Ms Ardern's star quality to grow and shine through.

And now you have a straight personality race with the latest polls showing her neck and neck with Mr English in the preferred prime minister stakes.

Some will argue that all the latest polls show is that there has been a reshuffling of the cards among the left and that National has retained its core support.

But anyone in politics knows that momentum is key and Ms Ardern now has six weeks to show what she is made of. If she shows that she is just too inexperienced and can't handle the pressure, then National will get a late boost to propel it to a fourth term.

However, if she appeals to voters in the televised debates by making Mr English look wooden and boring in comparison, then she probably will get National voters to jump ship.

If she does, prepare yourself for a Labour-NZ First government.