No sooner has a party got on top than it has to work out how to stay on top. That requires constant replenishment. A durable government needs a strategic cabinetmaker.

Contrast two episodes from history.

By 1949 the first Labour Government was in the last of its 14 years in office. Its top six ministers had all been there since 1935. Prime Minister Peter Fraser could not bring himself to fire old comrades. It was a visibly tired and hidebound government.

In the mid-1960s Prime Minister Keith Holyoake brought five vigorous new ministers off his back benches into his Cabinet of 16. They included a future Prime Minister, Sir Robert Muldoon, and Deputy Prime Minister, Duncan MacIntyre, and three who made third, fourth and fifth respectively in later Cabinets, Peter Gordon, Lance Adams-Schneider and David Thomson.

That injection of energy may have made the difference in the close-run 1969 election, in which Holyoake won a fourth term. It certainly provided the vitality for the National Government, which won back office after a one-term absence in 1975.

A government wastes away if not replenished. For that it needs talented people on its back benches. So it needs to pay close attention to its candidate selections, especially for the election that brings it into office or the one before.

Three of those Holyoake promoted in 1966 entered Parliament in 1960, one in a byelection in 1959 and one in 1963. By 1966 they were well blooded in political combat and backroom policy work.

So to provide for replenishment of National's Cabinet in 2011, if he becomes Prime Minister at this election, Don Brash needs a top intake of new MPs.

Two of the certain new MPs are top flight: World Trade Organisation negotiator Tim Groser and lawyer Chris Finlayson, who has a well-respected track record in the party's policy circles.

But neither is likely to be still outside the Cabinet in 2011. Groser is talked about as going straight into the trade portfolio in his first term and Finlayson to be fast-tracked.

Two other certainties, Jonathan Coleman (38), likely to win Northcote, and Guy Nathan (35), standing in Otaki, look to have what it takes. But beyond them the list is more worthy than whizzkid.

Still, on one score Brash is a match for Holyoake. Just as Holyoake had an able deputy, Sir John Marshall, who could succeed him (and did - but too late to win the 1972 election), Brash has super-fast learner John Key on track to the leadership, with some promising potential deputies in Katherine Rich and Simon Power as they mature.

Also in the wings is Bill English, who has yet to reach his full potential, despite being leader from 2001-03.

And, if Brash is forming a Cabinet this time, he should be able to present a capable senior echelon, though those in it would take time to get on top of their briefs: Key, English, Rich, Power, Nick Smith and Judith Collins.

The first four are all in their 30s or early 40s and in tune with a like-aged generation, which expects the service it gets from the government to be as flexible and customised as what it gets from the private sector.

Contrast Helen Clark's predicament. She has reached Holyoake's six-year point and, if she gets a third term, risks a "same-old, tired-old" team image in 2008.

Clark, like Holyoake, has a top-class deputy, Michael Cullen. Clark has an able top team - Phil Goff, Steve Maharey, Annette King, Pete Hodgson and Jim Sutton.

Holyoake had the quicksilver Ralph Hanan and bluff Tom Shand from the 1950s Cabinets (who both died in 1969) and a rising young star, Brian Talboys (1957 intake), who was to be Deputy Prime Minister to Muldoon.

Clark's top echelon has given her Cabinet impressive cohesion and strength through its first two terms, as Holyoake's top ministers did in his first two terms.

But Clark's are all from the 1984-90 government era and most in their 50s, which places them across a cultural and behavioural divide from the "customised" post-Rogernomics generation.

And, unlike Holyoake, Clark does not have abundant talent on which to draw for replenishment at this point. The 1990 intake included Maharey, Hodgson and the once-promising Lianne Dalziel, but intakes since then have owed more to loyalty in the awful post-Rogernomics years than to star quality.

The 1999 intake was not a patch on Holyoake's 1960 intake. There were no Muldoons or MacIntyres or Gordons.

It did produce (at her insistence) John Tamihere, whom she fast-tracked into the Cabinet in his second term and looked set for a senior role in this coming term but imploded late last year and then self-immolated this year. Tamihere will need a term or so to come right - if he can.

David Cunliffe, another Clark protege in 1999, is bright, able and a modern Labour type of Key's age. He could make the front bench eventually but has yet to develop political personal skills to match his ability. Clayton Cosgrove has potential above his status. David Benson-Pope is a capable fixit man but perhaps a bit too fast at it.

The 2002 intake brought in Russell Fairbrother, well-regarded by some in Labour's inner circle but with mixed reviews outside it. And even of those set to come in this election, only Shane Jones is potential star material - and he won't be a real force until after 2008.

Some potential stars in a term or two have not made the cut, for example, Stuart Nash in Epsom.

Of course, there are the minor parties. Peter Dunne, veteran of spells in the 1990 Labour cabinet and 1996 National one, is hardly new material. A couple of Greens would add an element of freshness - and surprise. But that is not a substitute for regeneration from within her own party.

Which brings us to Clark herself. Brash's succession is not an issue. But there is no successor to Clark. Cullen is entering his last term and doesn't want the top job anyway. No one else comes close - the nearest are Maharey when he recovers his equilibrium after his wife's death last year and Goff, but neither is a Key. A test of Clark over the next three years is whether she can do some succession planning.

The danger for Labour if she doesn't is that the party disintegrates when eventually she goes.

Why is replenishment so important to Clark now and Brash in a term or two? Because, as the first article in this series noted, the big game between Labour and National is to be the party that dominates governments over the next while, sets the tone and language of politics and makes our society over in its image.

A government that cannot replenish itself in office rules itself out of that prize. A sobering thought for Clark if she does pull off a third term.