If there is going to be one indelible image left by this presidential election it must surely be the image of Derek Nally's stabbing forefinger. It was aimed at a heckler in the audience of Questions and Answers who had accused him of "collaborating with the enemy". Mr Nally, his voice suffused with passionate conviction, responded: "If I was to lose every vote in this presidential election I would still say murder is murder." Derek Nally draws definite moral boundaries in every issue he approaches: Northern Ireland, crime, unemployment. For him Sinn Fein is not the same as the SDLP, the criminal is not the same as the victim, a man without a job has a harder time than a man with a job. Derek Nally believes that the most important quality in a president is the power to give good example.

Drawing moral boundaries is not an afterthought, but the very substance of the campaign. That is why he took up Mary McAleese so strongly on a report in the Sunday Business Post which suggested that she was, at the very least, soft on Sinn Fein.

The document which formed the basis for the report was drawn up by an experienced diplomat, Dymphna Hayes. Far from being written in a hostile or mischievous fashion, it is clear from its content that she is close to Mary McAleese.

Her superior, Sean O hUiginn, the then head of the Anglo-Irish section, has strong nationalist views. Mr O hUiginn and Ms McAleese do not diverge widely in their political perspectives, and therefore it is illogical that Ms Hayes or Mr O hUiginn would seek to damage Ms McAleese in any way.

In short, this comes across as a conversation between good acquaintances, reported to Mr O hUiginn, a man in sympathy with Ms McAleese, and who values her opinions highly.

Against that background, given a reputable civil servant reporting to a reputable head of section in a memo that was leaked to a reputable journalist, to any sensible reader it all rings true.

The thrust of the story in terms of a presidential election is that Mary McAleese, in an unbuttoned private conversation, expressed views which are strongly sympathetic to Sinn Fein at a time when they were still carrying on a terrorist campaign.

There is no sign of the Father Alex Reid/peace process context which she made so much of on Questions and Answers. The key sentence, "She seems sympathetic to Sinn Fein", clearly comes from a conversation about Ms McAleese's general perspective on Northern politics.

Had there been any Father Reid dimension in the document it is inconceivable that a journalist as diligent as Emily O'Reilly, who wrote the story, would have failed to mention it.

But there is more. The memo goes on to point out that Mary McAleese sees no point in "participating" in the upcoming elections in any "shape or form" in the absence of an SDLP/Sinn Fein "joint election platform".

Whether she means that she herself sees no point in participating personally, or whether she sees no point in her fellow nationalists participating is not clear from the context. But on either reading it is clear she wants Sinn Fein in the package. That does not support her contention that she has always been a strong supporter of the SDLP.

Finally, and most coercively, there is the positive tone and content of the last paragraph, where she seems socially comfortable in the company of Adams and McGuinness coming back from London, telling us both of them were "in great form". I cannot think of Derek Nally being comfortable in similar circumstances. At the very least he would ask for a change of seats.

So what does she have to say about all of this? She said it was a "spin", "out of context", and about a conversation she was having with Father Alex Reid about the peace process. Confusingly, she then denies that Dymphna Hayes reported her correctly.

All of this is very reminiscent of the classic defence in a defamation case: "I did not say these things and, if I did, they cannot bear the meaning the prosecution is putting on them."

Derek Nally believes that Dymphna Hayes, an experienced and reputable public servant and confidante of Mary McAleese, gave an accurate precis of a private conversation. Derek believes this document was leaked by someone who thought the public should know that, at a time when a terrorist campaign was in full swing, a candidate for the Presidency of Ireland was making sounds sympathetic to Sinn Fein. To Derek Nally, that person is a "whistle-blower", like Derek himself, a person acting from the highest motives.

Derek Nally does not accept the McAleese denial. She has made too many denials and back-turns, about the relationship with Charles Haughey, about her views on the EU, about her views on Northern Ireland as an "archetypal police state", for her denials to be accepted at face value. What Derek Nally said on Questions and Answers still stands. If she wants to clear her name she should sue.

John Caden, a former producer with RTE Radio and a former controller of programmes with Radio Ireland, is director of publicity for the Derek Nally campaign