Moves made in the past few days by two minor parties in the election campaign are significant not just for this election contest, but quite possibly for the way future elections will be decided.

New Zealand First and United Future, have both declared they will talk terms with whichever of the two big parties wins the most votes. While that may seem merely a reasonable and proper position for them to take, it is not the way politics always works in countries that have proportional representation.

In some parliaments of Europe it is common for the party with the largest number of seats to be sitting in opposition to a governing coalition of less popular parties. That possibility has been entertained by the Prime Minister, Helen Clark, who has pointed out her party has more potential partners than National.

New Zealand has had only three elections under MMP so far. Each of them has produced a Government led by the party that won the most votes, though Winston Peters held negotiations with both parties after the 1996 election before deciding to go into power with National. So nobody knows how the public would regard a Government that excluded the leading party.

It seems likely that an electorate long accustomed to first-past-the-post would take less kindly than Europeans to the idea that second place can win the prize. A Government consisting of the second and minor placings might struggle for legitimacy in New Zealand eyes and confidence in the electoral system could be lost.

Conventions are as important as electoral law to the country's political contentment and stability. If a convention develops that a small party winning a pivotal number of seats should always talk first with the party with the most seats, it would be a welcome development of MMP.

If that party is to the right of National or the left of Labour there might not seem much point in talking to its polar opposite. But even then, the third party might find it politic to first approach the party that has the support of most voters. The system is still shaking down in this country, but it can be expected to develop in ways that reflect the history and culture of politics here.

But that is for the future. As of now, the decisions of United Future and NZ First have changed the complexion of this election. The contest has been rendered wide open, not only in the polls but in the possible Governments on offer to voters.

Labour has only itself to blame for losing the advantage in terms of potential partners. By treating the Greens as its most likely ally in power, Labour has alienated Peter Dunne's party and driven him into the arms of National.

There he has found a welcome. National had been feeling so isolated that momentarily it even relaxed the cold shoulder it has presented to the Act Party. National, if not its leader, would much prefer to be associated with a party in the middle-ground and it has now declared United Future to be its preferred coalition partner.

Mr Dunne may be no more inclined than Mr Peters to take his party into a formal coalition with either party, but both have made it clear they will offer support on the vital issues of confidence and supply to a Government formed by the party with the most seats.

For voters the move by the two minor parties means there is little room now for tactical voting. Supporters of Labour or National cannot throw their party votes to their preferred coalition partner since they need to ensure their major party finishes first. The loss of tactical votes would be less costly to the parties of Mr Peters and Mr Dunne, who each hold electorate seats, than it would be to Act and the Greens.

This election is looking increasingly like the two-horse races of old when votes, not post-election deals, were decisive.