With Act having gained no traction since former National leader Don Brash ousted Rodney Hide as the party's boss, Epsom is a key seat. Brash has been so hamfisted, the 5 per cent party vote threshold needed to take seats under MMP is a distant target for Act.
And now its only other way of getting back in - Banks winning Epsom and dragging through a couple of cling-ons, including Brash - is under threat.
Admittedly, there are still a massive number of undecideds in Epsom - some 40 per cent - but Brash's cannabis warblings and the fact Banks was trounced in the Auckland mayoralty last year mean this poll is the clearest sign yet that they should be worried about their electability.
Key's other partner this term, the Maori Party, will also not be the force it has been post November 26. Te Tai Tokerau will stay in the hands of firebrand Hone Harawira's party, and it can't guarantee holding off Labour in the other Maori seats it has.
So what does Key do? His first ambition will likely be a World Cup win to Richie McCaw's men, a couple of knighthoods handed out quick-smart, and a rush to the polling booths in the hope National wins enough of the vote to govern alone.
That hasn't happened, though, since MMP came in back in 1996.
It leaves Key - and voters - some interesting choices in the coming weeks. Does the Prime Minister drag Goldsmith from the race or publicly encourage Nat supporters to vote Act? Does Epsom even want Banks after having Hide taken off them?
Key will probably trust his political skills in being able to manage a diverse set of hangers-on, if it happens. He has managed Act, the Maori Party and United Future's Peter Dunne remarkably well in the past three years.
But the Greens, the Mana party and even, heaven help us all, NZ First would be a much more volatile mix.
It's why readers shouldn't be solely focused on the rugby in the coming weeks. Do you want Key to govern alone, and do you think that is even possible? If not, who will you team him up with using your party vote?
The way the minor parties, Opposition big-fish Labour included, forced National to make concessions on its ill-conceived Video Camera Surveillance (Temporary Measures) Bill this week again showed the benefits of a system of proportional representation.
Having others with their hands on the nation's steering wheel moderates the more unpalatable ideas of the dominant party.
It's why the referendum on the future of MMP being held in conjunction with the election is also vital. It is hard to see us letting go of proportional representation now - but until today's poll it was hard to imagine a Parliament without Act. Get your thinking caps on.
