But the wider public has yet to apply the necessary pressure to leave Key with no choice but to strip Banks of his portfolios. There is no discernible public anger at what continues to unfold. The public seems largely unbothered by the allegations.
That may be because people are not surprised Act's leader has got himself into this mess. Banks and controversy are life-long partners. And, moreover, he is now stewing nicely in that mess, thank you very much.
This absence of any real public pressure for Banks' head appears to have persuaded the Prime Minister that he can tough it out and keep Banks on as a minister.
That is a big gamble. How long National can sustain the saturation coverage being given to the story is a moot point.
The collateral damage - if any - will be to Key's reputation if he is seen as not displaying leadership.
But Key has carefully laid out the parameters within which he will act if further revelations force him to review Banks' status.
Were Kim Dotcom, for example, to produce a tape or transcript revealing that Banks had phoned him to thank him for his supposedly anonymous donations, the Epsom MP would be in deep trouble.
Key will not sack him for not breaking the law. But he will ditch him if it turns out Banks' assurances of no wrongdoing are found to be worthless or that Banks has simply misled him.
Key has copped flak for seemingly applying a different standard to Banks' behaviour. However, the claim that Key is treating Banks in a more lenient fashion than he would ministers from his own party does not entirely stack up.
The resignations of Nick Smith and Phil Heatley followed clear errors of judgment on their part. Richard Worth' s departure - though never fully explained - seems to have been a clear breach of the standards expected of a minister.
The claim of inconsistency might apply in the case of Pansy Wong. She resigned from the Cabinet before the evidence to justify her going was produced.
Banks has escaped a similar fate because he is in a very different category.
Key would relieve Banks of his portfolios with the utmost reluctance, not because of any personal affinity he might have with Banks, but rather because Key's priority is to avoid doing anything which might fracture or weaken National's three-way confidence and supply arrangement which, apart from Act, includes separate agreements with the Maori Party and lone United Future MP Peter Dunne.
That is essentially Key's bottom line. National places huge stock on selling itself as the party of stable government. If maintaining that image means sacrificing strict adherence by ministers to the highest ethical standards, then so be it. Key is willing to pay the price and use up some of his political capital for National's greater good.
He is not going to sit back and give Labour the satisfaction of of watching part of his governing arrangement disintegrate.
In renewing its confidence and supply agreements with Act and United Future, however, National has had the lingering worry that both parties are one-MP parties. What would happen if Banks or Dunne fell under the proverbial bus?
In Banks' case, the bus has arrived early and come perilously close to running him over. Following this week's bizarre twists and turns, who would put money on the bus not reversing right over him?
As it is, the police's track record in investigating electoral transgressions would have had Banks twiddling his thumbs on Parliament's backbenches for months had Key stood him down as a minister this week.
Banks would still have cast Act's single party vote in National's favour. Not to do so would fly against his own party's principles, as well as incurring the wrath of Epsom voters.
But Key's job is to foresee the unforeseeable. Lingering in the background is the fear that - as has often happened with demoted MPs - Banks might go feral on National and start voting against the Government.
It is virtually inconceivable that he would do so, especially with his antipathy to Labour. But the possibility, however small, cannot be ruled out. So why take the risk? Especially as Banks' vote is pivotal to National passing the Mixed Ownership Model Bill, which paves the way to the partial privatisation of state-owned energy companies.
Were Banks to find himself marooned on the backbenches, he would be more likely to resign from Parliament and force a byelection in Epsom than play the maverick.
Such a byelection would return a National MP, restoring National's ability to pass measures not supported by the Maori Party.
It would also solve another problem. Banks' behaviour over the past week is hardly going to endear him to Epsom voters come the 2014 election. National may now have little choice but to turn off the life support system that has kept Act in Parliament.
Act has hit absolute rock-bottom in recent polls. The likelihood of the party bringing more MPs into Parliament on Banks' coat-tails would have to be rated at zero.
Its value to National is now reduced solely to Banks' vote in Parliament.
So far Key has insulated National from Banks' troubles. But he needs that vote.
Act is now not only the dead parrot of Monty Python fame.
Should there be more revelations that cast serious doubt on Banks' version of events, it could turn into a rotting albatross around Key's neck.
