Phil Goff took over as interviewer at times and generated the news headlines in tonight's final TV debate. But a measured John Key stood firm and calm as he rammed home his anti-debt message

Phil Goff had the details and the studio craft, but after a nervous start John Key had the authority. It's one of the things that a Prime Minister gains simply by going into work every day - and Key got the tone right to edge Goff in tonight's debate. Not that the legacy of his night's work won't have ramifications.

Going by his most repeated lines, John Key's central goal tonight was to drive home a fear of debt and the perception that centre-left parties borrow to spend irresponsibly. The message was clear - to borrow now is to put your mortgage at risk.

In many ways, the big policy stoush of this campaign has been how we afford to keep spending in tough times; where do we get the money. National wants to sell to raise the cash, Labout wants to borrow. Which is better in the long-run. Discuss.

Now I don't know anyone who, as Key claimed, has seen their mortgage repayments halved in the past three years. But all he wanted to do was stress the point of low interest rates and suggest that a Labour government would put that at risk.

Goff failed to deliver the most obvious rebuttal - that the biggest risk to interest rates rises was a credit downgrade, and who's presided over one of those? But Goff did offer more specific policies - on apprenticeships, the $5,000 tax-free zone, paying the dole to employers for a year and so on. He offered a lot of positives, whereas Key often turned negative.

Goff's strategy was to turn interviewer - but he never got in the killer question, as Key did at the Christchurch Press debate. Having said that, Goff's questions drew out fresh information, which is quite an achievement at this stage of a campaign.

Most tellingly, he pressed Key on only selling state asstes to New Zealanders, and the news that should be on the front pages of the newspapers in the morning is that Key gave his guarantee - a "commitment" - that 85-90 percent of the assets sold will stay in New Zealand hands.

Whether he meant 85-90% of the whole or of the 49% to be sold, that's still something new. It's also a promise that's impossible to keep, unless he puts legal boundaries around the sales. And if he does that, the price goes down and the $5-7 billion is less likely to be achieved.

Of course, even if the initial sales go 85-90% to New Zealanders, that doesn't stop shares being sold offshore the next day or the next month, but even that commitment made tonight is bold. Some might say impossible to keep.

Like closing the gap with Australia, that will be a stick to beat him with for years to come.

As will the way he's turned on the Greens this week. Sure, he wants to whack Labour, but by including the Greens in his attacks, he's damaged what might have been a burgeoning relationship. Again tonight he labelled them as 'the other'; not his kind. If the Greens stabilise as a clear third power in parliament he may come to regret that tactic at the next election.

Key was also loose when he talked about the Maori seats as an "arcane thing of the past". There's some debate as to whether he meant "archaic" or "arcane", but either way Tarian Turia will have been sitting in her armchair fuming. That wasn't a "mana-enchancing" comment, nor good for relationships. The Maori Party will have to express outrage at that, or else Mana will use it to paint them as a National Party stooge.

The other newsgrab from the night, I'd hope, is Goff's claim that National has told senior police officers to rein in recruitment, but not let anything out until after the election. Newsrooms should be onto their police sources now, because that's something either way - either the Nats are up to something or Goff is bleated without being able to substantiate. When National talked about its state sector cap, police and prisons were excluded, remember. Has that ring-fence been pulled away?

Key was wrong to accuse Goff of 'rewriting history' over crime stats. Goff was exactly right that crime has been trending down since the mid - or even early - 1990s. And Goff's studio craft - turning to Key, calling him by name, challenging him with questions - were well played.

But Key kept is cool with a measured, clam performance. He was strong on asset sales, he spoek strongly about creating high-paying jobs and would have won support for his welfare reform ideas.

Undecideds would have been split tonight, I suspect and both men would have shored up those already inclined to vote for them. So while it created some great headlines for the next 48 hours, the debate didn't change the broad thrust of the campaign. And that's to Key's advantage.