For John Key and National, passing the credibility test has suddenly but, not unexpectedly, become a far tougher assignment. The bar Key must clear has been raised several notches.

This is not solely because Labour is increasing its already frantic efforts to label National's leader as too shallow, too lacking in principle and too inexperienced to do the job of Prime Minister.

As the latest TV3 poll indicated this week, an overwhelming majority of voters _ including a fair chunk of Labour ones _ expect National to win this year's election. Key has transitted from new boy on the block to Prime Minister-in-waiting.

That is the standard by which his performance will increasingly be judged by voters from now on.

At the same time, the media are acutely conscious of repeated complaints from Labour's side of the fence that Key is not being subjected to enough scrutiny; that he is getting an easy ride into Premier House; that his positions on issues change from day to day depending on his audience; that he is just too good to be true; and that there must be a skeleton somewhere in his past.

Finding one would almost be a relief. The parliamentary media's fear is that once he has been sworn-in as Prime Minister, he will reveal his true self and start driving through a completely different agenda to the recipe of moderate conservatism he has cooked up in order to win power.

Judging from everything Key has said and done so far, that seems unlikely.

However, the inevitable post-election Labour chorus of "we told you so" means Key is going to be put on the spot at every opportunity beforehand.

That was the case on Tuesday when reporters grilled him on his way into the House. With the Government having just put the kibosh on the sale of 40 per cent of Auckland Airport to Canadian interests, the obvious questions were what would National have done had it been in government and _ if it gets to be in government _ what will it do about Labour's tightening of the criteria covering the sale of so-called "strategic assets".

Previously, Key might have got away with a "we're still working on the detail of our policy" type response.

However, the sudden prospect of the Canada Pension Plan Investment Board getting sufficient shareholders to accept its offer plus the Government's pre-emptive strike against that happening meant matters had become too pressing for Key to fudge.