The Green Party has been in Parliament for 15 years now and is the only party never to have been part of a government. That is not entirely the Greens' fault. Twice Helen Clark preferred to form partnerships with parties to her right rather than her left, which is strategically understandable. A rival on the same side of the fence is a greater threat.

At this election Labour is at a low ebb and some of its usual supporters appear to be considering a vote for the Greens. In last week's Herald-DigiPoll survey Green support rose above 10 per cent for the first time in nearly a decade. And that was before the party formally launched its campaign yesterday.

The campaign has three weeks to go and much can change. Previous experience suggests the gap between the major parties will narrow and National will not finish with the majority it still has in polls. History suggests it will need the support of at least one other party to govern for a second term. With Act struggling in Epsom and the Maori Party likely to win fewer seats this time, the Greens may be pivotal.

Already, they have indicated one item they would want in a post-election deal and it is one that National would probably grant. Country-of-origin food labelling is not too fearful a demand. It is not particularly wise for a country that exports food to encourage xenophobic sentiment anywhere, but it does not sound like a deal-breaker.

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In fact, it says the Greens are open to a deal. They could have nominated more unpalatable policies if they still preferred the safety of the sidelines to a compromise with a political enemy. The party's personnel has been changing at each election. This time it farewells Sue Kedgley and Keith Locke, last of the originals.

Under co-leaders Russel Norman and Metiria Turei the Greens have presented a more businesslike face. Mr Norman is well versed in economic policy and Ms Turei is an engaging communicator. If they can win 10 per cent of the vote they could command 13 seats in the House. Only one sitting MP Catherine Delahunty has threatened to resign if the party does a deal with National.

But she probably represents a significant constituency of Green supporters. Polls show the party's support is predominantly younger voters, who may be less willing to compromise and entertain hopes that the Greens might one day govern alone. Many of them might always prefer the purity of the sidelines to the decisions required of a party in government.

Green MPs will be alert to the risks they would face in partnership with either of the main parties. They have been in Parliament to witness the demise of the Alliance, New Zealand First, Act and quite possibly now the Maori Party. All have struggled to distinguish themselves from their major partner, at least in voters' eyes.

But the Greens might have a more discerning constituency than the others and their MPs might be better able to distance themselves from decisions they do not like. There is no reason that a small party should be tarred by everything done by the party that it enables to remain in power. The country needs a government and it is properly the party that has won the most votes. A third party's support for it on confidence and supply can mean no more than recognition of its right to govern.

National conceded a few policies to the Greens in its first term, notably subsidised home insulation. They will probably aspire to more than food labels if National needs their support. They might even get agriculture back into the emissions trading programme.

The Greens should seize this opportunity, let the electorate know they are ready to graduate from perennial opposition to become a serious, practical force in the nation's affairs.