It will have to fall a long way. You have to go back to National's nadir - the 2002 election - to find a scenario where New Zealand First and Labour combined outpolled National. That was in the party vote - not the electorate vote which still flowed in copious quantities to the ever-popular John Carter.
There is evidence that National is expecting to take a major hit, however.
Three weeks ago, Key was not giving Peters "any chance" of winning. This week the Prime Minister was talking of the "difficulties" and "challenges" in fighting a byelection as the ruling party even in supposedly safe National seats.
To back up his argument, Key cited the Selwyn byelection of 1994 which was triggered by Ruth Richardson quitting Parliament after Jim Bolger sacked her from the finance portfolio.
Key noted that her replacement - current Speaker David Carter - had just managed to scrape home in a seat which National now held with a 20,000-plus majority.
Key, however, was being selective with the facts. Richardson enjoyed a healthy 5000-plus majority following National's landslide victory in the 1990 election. That was slashed to less than 900 votes at the 1993 election. David Carter actually did quite well in keeping that majority largely intact at the subsequent byelection.
What Key was really doing was lowering expectations in advance of Saturday's ballot as to how well National will do.
What Key has not explained and maybe cannot explain is why National is struggling in a seat where it has long had a large majority and when - unlike in 1994 - the party is still receiving strong voter backing nationwide.
One answer may be found in the "theory of rising expectations". This notion argues that revolts, rebellions and revolutions do not happen at the bottom of the economic cycle. It is when things start to improve and expectations are raised only to remain unsatisfied or dashed completely that revolt occurs.
Northland is an almost perfect fit for such a scenario. The region's economy has grown in the wake of the global financial crisis - thus raising expectations. But that growth was at a far lesser rate than in other parts of the country.
The more National talks up the economic recovery, the more Northlanders feel they are missing out. And even more so with economists warning that provincial cities like Whangarei could end up being "zombie towns".
Then suddenly out of nowhere comes a byelection - the ideal vehicle for venting that frustration.
Who better to lead the ensuing rebellion than the country's most practised Opposition politician.
No other politician understands or caters for the conservative disposition of regions like Northland than does Peters.
He has so far run a near-faultless campaign. National has run what must be one of the party's worst. But there are things beyond Peters' control which may well have a significant impact on the outcome of the byelection.
First, a fair chunk of those seriously flirting with voting for Peters will still harbour some residual loyalty to National. Confronted by the ballot paper, many will do what they have always done - tick the box next to the National candidate.
Second, much hangs on National getting identified supporters to the polling booths. With National Party membership in the electorate among the highest in the country, there should be no shortage of volunteers to work the phones and ferry targeted voters to a polling place.
Third, Labour's candidate, Willow-Jean Prime, is still on the ballot paper. Although Andrew Little has given Labour supporters clear licence, though not clear instruction to vote for Peters, Prime will still pick up a large bundle of unwanted votes to Peters' cost.
Fourth, neither the Greens nor Colin Craig's Conservative Party are standing a candidate.
That leaves some 5000-plus votes up for grabs. Craig's share would logically switch to National. Green Party supporters would have to swallow hard to vote for either National or Peters.
Peters has sought to mitigate these factors by seeking to rise above the battle-lines drawn by political parties and focus on the one thing everybody agrees needs action - the dire state of the Far North economy and the consequential social problems.
His sales pitch for his candidacy tackles head-on the public's big worry of political instability by stressing a vote for him will not change the Government - but it will change how National behaves.
Peters can argue that is already happening. National's crude pork-barrelling has played right into his hands.
What is less obvious is the absence so far of incendiary speeches or policies on Peters' part. He is providing nothing which National can grab hold of and try to turn voters against him.
National has been forced to resort to attacks on Peters' credibility as a candidate by questioning why New Zealand First failed to put up one in Northland in recent general elections; that, if successful, he would still live in Auckland (something Peters denies); and that he will not commit to standing in Northland at the 2017 election.
This is really scraping the barrel, pork or otherwise.
National's big mistake is that it is fighting the Peters of old - the fire-brand maverick who was forever wedded to the politics of Opposition.
The Peters of 2015 is a far less polarising character. He is more relaxed. He is less prone to argument for argument's sake. His prime political weapon these days is humour. He is now tolerated - even respected - by those who previously detested him.
National's attempts to demonise him are thus falling flat. Voters now trust him much more than was once the case.
In contrast, trust in Key and National has slumped in Northland as a result of the paucity of information surrounding the police investigation of the electorate's previous MP, Mike Sabin.
And that is what this byelection is really all about. It is all about Peters punching a major hole in Key's seeming invincibility, while finally extracting revenge for Key's treatment of him as a political pariah.
