Key says the Treasury ran the numbers through its economic modelling this year and leaving super untouched is "affordable and do-able".
This is presumably the same Treasury that warned in April that keeping super affordable could require an increase in GST to 19 per cent in the early 2020s or everyone paying an extra $30 a week in income tax.
Key's problem is that even if he is correct, no one is going to believe him. Voters - certainly those under 40 - have become conditioned to the pension jar being empty by the time they retire.
Moreover, everyone knows that everything Key says on the subject has to be seen in the context of his promise to resign if super entitlements are changed during his watch.
Labour has exploited Key's straitjacketing of himself with Phil Goff trying to turn the issue into one about which party is displaying real leadership.
When it comes to credibility gaps, Key is not alone, however. National is being unrelenting in turning the screw on Labour and forcing that party to detail exactly how it will achieve the seeming impossible - bringing the Government's books back into surplus at the same time as National despite intending to spend more than National without the revenue stream to pay for it.
Labour's finance spokesman David Cunliffe yesterday took strong exception to National's running total of Labour's extra spending and how much it would have to borrow - $17.2 billion over four years at last count. He yesterday promised that a detailed rundown of the fiscal impact of Labour's promises was "not too far away".
Labour has been gambling that voters cannot be bothered to follow arguments about policy costings because those doing the sums will twist the figures to their advantage.
Yet, Labour knows that it can hold out only for so long. National's heavy emphasis on getting back to surplus as soon as possible no doubt flows from focus-group research of voter attitudes. The public mood is very debt averse.
Labour knows it must come up with the fiscal goods - and soon. Otherwise National's estimates - accurate or not - will fill the vacuum.