Key Points:

Can we trust John Key? The question has to be faced after his appalling slip on television this week. Most voters will have seen and heard his behaviour when a Television New Zealand reporter asked him directly how many shares in Tranz Rail he held in 2003. At first he answered falsely, then, realising he was walking into a trap, he corrected himself.

Voters are trusting people, particularly when they want to vote someone into power. Voters who want to put the National Party into office next month need to trust him. Need can colour judgment. They may be too willing to make excuses for him.

They may say TVNZ was well primed by Labour Party researchers to ask the question. That seems to be true but does not matter. Labour may have set the trap but honesty would have survived it.

It is also true that Labour no doubt timed the stunt to try to deflect attention from the privileges committee's decision on its ally Winston Peters that was made public that evening. That too is no doubt true, but the distraction was also valid. It applied to Mr Key exactly the same test he and others have asked of Mr Peters: were his interests properly disclosed?

In their efforts to excuse National's leader, some may say the size of his small shareholding five years ago does not matter; whether it was 50,000 shares as he initially said, or 100,000 as he conceded, is of no political importance. It is so immaterial to the issue that we can only wonder why Mr Key did not own up to the larger figure at the outset.

But then, we have been wondering why Mr Peters did not simply admit that Owen Glenn contributed to his legal expenses when he was asked whether Labour's biggest donor had also contributed to New Zealand First. Honesty looks harmless in hindsight.

Embarrassments such as these are not mere errors of political judgment, or presentational "gaffes". They are much more than "a bad look". There is no way to downplay the fact that when Mr Key was asked the question, his first instinct appeared to be to deny the truth.

He says it was a "genuine mistake". Television viewers may say it did not sound like it. This has to be deeply worrying for the electorate that will be voting in six weeks. Mr Key is not a bit-player like Mr Peters in the disposition of power. Mr Key stands to be Prime Minister, a position that carries enormous personal advantage in our system of party-government. The party leader has enough executive posts at his or her disposal to command a majority of a government caucus.

Mr Key now regrets holding any Tranz Rail shares in 2003 when he was National's transport spokesman and eliciting information on the company in Parliament and a select committee. He most certainly should not have been holding any.

He says he realised only after the issue had faded in 2003 that he had owned twice as much of the company as he had admitted. But that cannot excuse his prevarication this week.

Nor is it an excuse that he was not asked the question previously. That has been Helen Clark's reason for not telling what she learned from Mr Glenn in February. It is the sort of sophistry Mr Peters made his trademark. The public has a right to candid information from its representatives.

It may be that Mr Key committed the slip of a front-runner in the electoral race. Like many a sportsman on the verge of success, he has become too cautious, risk-averse, afraid to take an unpopular position, quick to backtrack from any policy implication that may be contentious. He has choked. We can but hope that is all it was.