• John Armstrong is a former New Zealand Herald political correspondent. He will be writing columns in the Weekend Herald from now until the election.

Don't dabble with New Zealand First. Don't smooch with the Greens. Ignore the minor parties if you are serious about wanting to change the government at next month's general election.

To optimise the likelihood of a change you must cast your party vote for Labour.

Likewise, if the retention of the status quo is your priority, you must vote National.

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The astonishing resuscitation of Labour from near cadaver status to not far off Cabinet status has turned the election into a dogfight between the major Opposition party and National to the exclusion of the minor players.

What is now at stake is something which has become an absolutely crucial, but little-recognised feature of MMP elections.

That component is the widely accepted view that the party with the largest share of the nationwide vote holds the advantage when it comes to having the first crack at forming the next government.

In the seven elections since New Zealand adopted a proportional representation electoral system, the party winning the largest share of the vote went on to form the next government.

So far, there has been no case of two or more parties forming a bloc and securing power by virtue of being able to outvote the party that has won the most seats.

It could happen. The Greens were counting on it happening this time.

A far more worthwhile prize is now not far short of being within Labour's grasp, however.

If Labour turns out to be the party which secures the largest share of the vote and New Zealand First has the numbers sufficient to provide Labour with a majority on confidence motions in the new Parliament, then Winston Peters' choice of partner will effectively be no choice.

Peters would be obliged to strike a deal with Labour which led to the formation of some form of two-party governing arrangement, be it a coalition or otherwise.

If Labour still needs the Greens on board, that could give Peters room to wriggle out of Labour's grasp and reinstall a National-led Administration.

The upshot is that a vote for the Greens is now akin to a wasted vote for those who want a change of government.

Worse, it would be a completely wasted vote if the Greens fall below the 5 per cent threshold.

Labour still has ground to make up to overhaul National. Thursday's 1 News-Colmar Brunton poll had the current ruling party securing 44 per cent of the vote against Labour's 37 per cent.

Whether Jacinda Ardern can lift her party's support even higher and pick up the extra three or four percentage points to put Labour on course to become the party with the most seats in the next Parliament is information to which only the political gods are privy.

What is less in question is that backing for National is on the slide, albeit a slow one.

There are a lot of "ifs", "buts" and "maybes" in all this.

If the polls are telling us anything, however, it is that we are witnessing the re-establishment of what some might call the Old World Order and others would slam as the Two-Party Tyranny.

National and Labour would not care a jot about that.

Rather than enduring minor party stabs in the back, they could concentrate on doing what they like doing best, namely knifing each other in the front.