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Had David Cunliffe known the answer to Mr Key's attempt to catch him out on the detail of Labour's proposed capital gains tax, then Mr Key would have looked a bit silly - especially as the answer he had was wrong.
But Mr Cunliffe did not know the answer to Mr Key's question.
In an instant it was obvious the Labour leader was floundering.
On such moments election campaigns can turn.
This was one those moments.
Mr Key took a punt. But it was thought to be a pretty safe one.
National believes Mr Cunliffe does not bother to get on top of detail.
For him, a capital gains tax is a means to satisfy the Labour left.
Mr Key's question more than did the trick.
It shifted the debate about the tax from arguments about its benefits in terms of fairness and shifting money into productive investments to questions about detail and its impact on middle New Zealand.
As one sage observes of Labour's crusade for such a tax: Everyone likes the idea, but no-one wants to pay it.
National had a field day for the next 48 hours, highlighting anomalies and contradictions in Labour's proposal.
Mr Cunliffe managed to prolong the firestorm by getting it wrong again, this time on how the tax would apply to a home inherited from dead parents.
Labour should have foreseen National's onslaught. It should have devised a strategy in readiness to counter it.
But Labour may have been lulled into a false sense of security.
Labour's intention to implement such a tax was part of its 2011 manifesto.
But National did not aim its heavy artillery at the tax at that election.
Labour may also have thought it had already won the argument by virtue of a broad consensus in the financial sector that such a tax is long overdue and - as a result - National is the one which is out of touch and the pressure has been on Mr Key and Bill English to come into line.
Beyond issuing a 10-page backgrounder which added little that was not already in the 2011 manifesto, there was no briefing, press conference or attempt by Labour to spin its way out of this disaster.
Already struggling to make any impact, Mr Cunliffe's campaign has taken a potentially fatal knock.
With two opinion polls yesterday showing Labour marooned at about 25% and National registering at 50% or more, the election campaign may effectively be over.
The only question now is whether growing backing for Colin Craig's Conservative Party will translate into actual votes on September 20 and in sufficient numbers to clear the 5% threshold - and thus allow Mr Craig to come to the negotiating table with enough seats for National not to have to entertain a deal with Winston Peters.
While it is too late to gift Mr Craig an electorate seat, it is likely Mr Key will make some carefully worded statement in the final week of the campaign giving licence for potential National voters to tick Conservative.
The possibility of Labour being in a position to form a government now looks to be virtually non-existent.
With two weeks still go, Labour's campaign is The March of the Living Dead.
• John Armstrong is The New Zealand Herald political correspondent.