Imagine a long-distance runner crossing the finish line and grabbing a cup of water. The first cup is magical. It gives maximum benefit. The second is almost as good. The third, as thirst begins to abate, is marginally less satisfying. And so on, until at a certain point one more cup produces no benefit, or even unfortunate consequences. That concept is similar to economics' Theory of Diminishing Marginal Utility.
In democracies like ours there comes a time when the political cycle demonstrates its own law of diminishing marginal utility - and an electorate once refreshed but later sated requires the two main parties to swap roles between government and opposition. For policy, ideas and leadership, that change can be as necessary as it is inevitable; refreshing the body politic each time it occurs.
If all governments end in failure, then in the New Zealand context those failures since 1960 have come after 12, three, nine, six and nine-year administrations. Tomorrow, another government of nine years attempts to smooth the political cycle beyond recent norms and back to the days of Holyoake's National.
Labour under Helen Clark is pinning its hopes on the electorate acting counter-cyclically because of the global financial crisis; that in an age of such uncertainty, voters will collectively ignore a nine-year-itch for the new, opting instead for continuity.
National and its allies presume their time has come. Public polling for more than a year would indicate the pendulum is set to give the centre-right another chance to lead. The shape of any such government cannot be known until the vote count. Yet whatever the result, some important policies will endure. Our political cycle over the past generation seems not to lead to wholesale reversal, but selected change and a more general accretion of policies.
When Jim Bolger's National team swept to power in 1990 it adopted core initiatives of the Fourth Labour Government: economic deregulation, the Reserve Bank Act, state-owned enterprises, Tomorrow's Schools. Similarly, come 1999 and Helen Clark kept not only the past new-right innovations of her own party but also National's Fiscal Responsibility Act, reduced welfare benefit levels and the privatisation of Contact Energy and commercialised state power companies. As always there are differences: the Clark years have seen a distinct halt to private involvement in public services and a universality of subsidies and credits which might not be sustainable.
Should John Key's National form a government after tomorrow, many of the Fifth Labour Government's innovations will continue: the Cullen Fund, KiwiSaver, Working for Families. Mr Key has been painfully keen to inoculate his party against accusations of secret agendas to remove such popular policies.
The Prime Minister is aware how rare a fourth term would be. She argues that under MMP the old "best-before" dates do not apply to ever-changing governing groupings and that economic convulsions require New Zealand to put "trust", not "change", as the paramount driver of the poll. It may be Labour's destiny to join the Opposition benches but under her new paradigm that is at least three years away.
The economic context of this election could not be more challenging. New Zealanders need to deliver a decisive mandate, one way or the other, so that responses to the recession and credit crisis can be emphatic. Second-guessing MMP by flanking major parties with second-tier partners might not be the best answer now.
The "ask" is simple: Is there still benefit for the country in drinking one more time from the cup of Labour, or is another source, National, needed now for economic and political revival?