It is thus Key's very good fortune that the two such ballots taking place this year - one in the late Parekura Horomia's Ikaroa-Rawhiti seat and the other in the Christchurch East electorate of mayoral aspirant Lianne Dalziel - are in what have traditionally been safe Labour seats. That sees the blowtorch pointing directly at Labour's leader when the votes are being counted.
With National's name absent from the ballot paper in Ikaroa-Rawhiti, the biggest worry for Key next Saturday will be how well or how poorly the Maori Party does and whether that has any ramifications for National's relationship with the troubled party. Meanwhile, Key has been making mischief.
This week, he predicted the contest in Horomia's old stamping ground would be a much tighter affair than most expect. Even if Key's claim is based on National's own polling, the prediction still has to be taken with a large dose of salt.
Obtaining reliable figures from polling in the Maori seats is notoriously difficult.
The Prime Minister's talk of a close result has at least three purposes. The first is to imply Horomia's near 6,500-vote majority means Labour should win easily, thus upping the pressure on Shearer to get a good result. The second is to put the frighteners on Shearer by hinting National knows more about the likely voting patterns than Labour does. The third is to persuade non-Labour voters that they should not assume the incumbent party is going to cruise to an easy victory - and should not waste their vote by staying home.
The worst outcome for Key would be resounding, morale-boosting victories for Labour in both seats.
Three factors are likely to deny Labour such a triumph in Ikaroa-Rawhiti: The relative absence of anti-Government sentiment looking for the lightning-rod of a byelection to manifest itself; the loss of Horomia's personal vote-pulling power at the ballot box; and voter turnout in the Maori seats, which is bad enough at general elections and truly abysmal in byelections.
Turnout plummeted to less than 33 per cent in the 2004 Te Tai Hauauru byelection called by Tariana Turia to validate her switch from Labour to the Maori Party.
In the preceding 2002 general election, turnout in that seat was close to 60 per cent.
The poor turnout at that byelection could in part be explained by other parties opting not to stand candidates.
The same could not be said for the 2011 byelection in Te Tai Tokerau. That was another example of an MP - in this case, Hone Harawira - precipitating a plebiscite to obtain a mandate for switching parties.
The difference was Harawira faced stiff competition from the Labour candidate.
It was an absorbing, high-profile battle.
Yet turnout barely managed to top 40 per cent compared to 63 per cent at the preceding general election.
These factors may reduce the scale of Labour's victory In Ikaroa-Rawhiti. But that victory should still be clear-cut.
Bedevilled by ongoing leadership ructions, the Maori Party is focused more on its own problems than those in Maoridom. Harawira's Mana seems to have lost momentum and drive since the last election.
The Greens' candidate will likewise have a struggle make any dent in Labour's vote. Allowing for turnout, anything much less than a majority of around 2500 to 3000 will see raised eyebrows in Labour circles and questions being asked of and about the leader.
A poor result will also up the ante for Shearer in the Christchurch East byelection which will most likely take place in November after the previous month's local body elections, which themselves provide a rough litmus test of national sentiment.
On one level, holding Christchurch East should be a relative cake-walk for Labour. National has performed one of the great political miracles in stopping post-earthquake frustrations from boiling over into the wider political milieu.
But signs that patience is wearing thin were evident in a recent Fairfax poll which had Labour rising to 40 per cent in Christchurch in terms of support and putting the party within touching distance of National.
Those numbers would be even more favourable in the less affluent, earthquake-blighted east of city.
On another level, however, Christchurch East provides shocking evidence of Labour's decline.
Between 2002 and 2011, Labour's party vote in the seat dropped from more than 16,000 to just over 9000.
In contrast, National's party vote soared from around 4,400 to more than 13,000, and - astonishingly for such a red-ribbon seat - overtaking Labour's share in the process.
Dalziel's majority is a third of what it was in 2002. In 2011, her majority was boosted by nearly a quarter of those who gave their party vote to National casting their electorate vote for her.
These voters are the lost tribes of Labour shacked up with National. They can be found in every electorate in number.
Labour will assume that 2011 was the high-water mark for National - and that from thereon the share of the party vote would begin to shift back in Labour's favour. The need to shift those figures underlines the necessity that Clayton Cosgrove be chosen as Labour's candidate. This is not going to be a byelection for political novices.
The perception is that National has nothing to lose in Christchurch East. It has a lot to lose and - from Key downwards - it will fight tooth and nail to halt any drift of voters back to Labour. Key's ridiculing of National's opponents suggests he has already begun.
In that respect, the byelection is going to be the real test of whether Shearer can step up to the mark and not only match Key, but better him.
