Once again Mr Key has trumped National's opponents and neutralised the political advantage they had held by jumping across the political divide and setting up camp in their territory.
He first did it with nuclear-ship visits when he became National's leader in 2006. He simply used his honeymoon in the job to declare the anti-nuclear law would remain intact under his leadership. And that was that. It may not have greatly impressed the Americans, but in an instant a political millstone had been removed from National's neck.
On numerous occasions since, Mr Key has likewise swallowed hard and taken positions which do not sit that comfortably with National ideology but which spike the guns of the party's enemies and leave them with nowhere to go.
Given the likely fine margin between victory and defeat at next year's election, nothing is sacred.
If retaining power requires the kind of monumental U-turn Mr Key has made on the City Rail Link, then so be it. No matter that he trod on the toes of Cabinet colleagues who had long pooh-poohed the rail ''loop'' on the basis of numerous experts' reports whose cost-benefit analyses placed big question marks over the worth of the project.
No matter that National has blatantly filched Labour and the Greens' policy.
With yesterday's confirmation of a tunnel as the second harbour crossing plus sundry motorway extensions and developments, Mr Key has mapped out National's vision for Auckland transport and, perhaps more importantly, laid out the stages by which that vision will be achieved.
In one swoop, he has taken the steam out of what, after housing affordability, is the thorniest issue in the country's biggest city - traffic congestion - and one on which, according to opinion polls, National's management has less than impressed the public.
In particular, Mr Key has now marginalised Labour and the Greens in the one aspect of public policy where those parties thought they safely had it all over National - public transport.
They have been left querying the 2020 start date for the City Rail Link project, saying it should be much earlier.
Mr Key, however, argues that the later date will see the work completed when patronage will be high enough to justify its construction. But he has left room to bring it forward if it becomes clear Auckland's CBD employment and rail patronage growth hit thresholds faster than current rates of growth suggest.
Apart from shoring up National's support in Auckland, the go-ahead is intended to remind the rest of New Zealand that National - unlike its opponents - looks at the big picture and gets things done whereas they are consumed by the relatively trivial, such as the fate of Peter Dunne and his parliamentary allowances.
For Labour - already wallowing in the Slough of Despond in the wake of its poor showing in the latest Herald-DigiPoll - the big tick for the City Rail Link was another reminder of how difficult it is to second-guess Mr Key.
The irony is that Labour had a head start. The whole transport package was due to be released yesterday. However, Labour's Te Atatu MP, Phil Twyford, got wind of National's switch in thinking on the City Rail Link and tipped off some media organisations. In hindsight, he and David Shearer might have been better advised to have called a press conference and challenged Mr Key to confirm National's sudden conversion to the wish-list of Auckland Mayor Len Brown.
Mr Shearer would have looked proactive and as if he had got one over Mr Key.
But missed opportunities and tactical ineptitude are bedevilling Labour.
Mr Key's legislation clarifying the powers of the Government Communications Security Bureau is a case in point. Labour has taken what on the face of it is a principled stand of demanding a full public inquiry into the role and functions of the intelligence agencies before determining whether its MPs will support the legislation.
Labour, however, has misread public opinion. While this week's Herald-DigiPoll showed majority support for an inquiry, that should not be misinterpreted - as some in Labour will do - as a desire for a curtailment of powers enjoyed by the SIS and the GCSB.
With a badly wounded Mr Dunne now vacillating over which way he will vote on the Bill, Labour's departure from its previous position of offering bipartisan backing to security laws has merely opened the door to Winston Peters and New Zealand First to fulfil that role.
Mr Peters says he understands where Labour is coming from. He likewise wants stronger and more transparent oversight of the country's spooks.
But he also sees it as his responsibility to ensure the two intelligence agencies are not handicapped by poorly written law.
As he said this week: ''You're not going to see a whole lot of mangled bodies at some airport or railway in this country because we haven't done our duty.''
Both Mr Key and Mr Peters instinctively realise that the public accepts a kind of trade-off in the post-9/11 world where no-one can assume they will not be targets of some act of terrorism.
The trade-off is acceptance of tighter surveillance and monitoring by the intelligence agencies and loss of privacy in return for assurances that it considerably lessens the chances of coming home from some overseas trip as a mangled body.
Mr Key has managed to get under Labour's skin by taunting Mr Shearer with the suggestion Helen Clark would never have gone soft on terrorism in the way he has.
Labour would have been wiser to claim the high ground by supporting Mr Key's Bill, but declaring it a stop-gap measure until a properly constituted inquiry had reported and permanent law flowing from the inquiry's recommendations could be drafted.
That would have marginalised Mr Peters and stopped Mr Key from using the law change as an opportunity to end the long freeze in National-New Zealand First relations.
The mistake would be to view a more co-operative relationship between the two parties as an indicator of Mr Peters' post-election intentions. For those trying to read what is happening now as a guide to what will happen next year, some advice: it ain't worth the bother.
- John Armstrong is The New Zealand Herald political correspondent.