Last month, a leading Holocaust denier, Dr Fredrick Toben, was arrested at Heathrow on a European arrest warrant. In court he complained that there was a witch-hunt mentality in Germany and that the Germans were out to get him. He appeared not to appreciate the irony of this accusation.

In the same way, it is hard to take entirely seriously the lamentations of the British National Party over the publication of its membership list. Having complained that the publication of the list opened its vulnerable members to violence and intimidation, Simon Darby, the BNP spokesman, announced that the person responsible for publication had made a serious mistake. “I wouldn't want to have done that,” he said. “I wouldn't be sleeping well tonight.” The juxtaposition of plea for sympathy and veiled threat was not, as with Dr Toben, intended to be ironic.

Nor was Nick Griffin, the party leader, employing irony when he announced that he believed that the human rights of his members had been breached and that he might use the Human Rights Act to seek redress.

The exposure of the British National Party's hypocrisy - appealing to a liberalism that it holds in contempt - is one reason to welcome the list's publication. A second is that it reveals that the party remains tiny and weak, despite its electoral success. Finally, the breach of security is likely to undermine and distract the party leadership at a time when it might be hoping to use the recession as a recruiting sergeant. Yesterday numerous party members were calling for Mr Griffin to resign.

Yet for those who believe, as The Times does, that liberty is the best defence of liberalism, satisfaction at the blow to the BNP should be limited. The breach of the privacy of members, with the publication of their addresses and telephone numbers, is regrettable. If a concentrated effort were made to prevent BNP members from earning a living that would be more regrettable still.

These concerns are not inspired by a naive belief that the purity of liberalism must never be sullied. They derive from a cool and practical assessment of the advantages to be gained from a defence of free association.

First, democrats gain when political arguments are advanced by open debate and settled by ballot. Extremists prefer threat and counter-threat. Far-left and far-right groups often seek to dominate politics by trying to ban each other and using violence to silence their opponents. In this way they feed off each other. Threats made against BNP members and by BNP members in the past 48 hours illustrate this danger.

Secondly, the introduction of a political test for employment might quickly become a serious blow against liberty. If membership of the BNP is regarded as grounds for dismissal, how long before the same is true of membership of other parties or of certain religious sects? In universities, bans that start as injunctions against members of far-right groups have often mutated to much more serious, and broader, threats to free speech than the far Right itself poses.

This potential for a creeping threat to liberty has to be balanced against the immediate harm that allowing BNP members to be employed in public service might cause. There have been moments in history - the American Communist Party in the 1940s, for instance - when a political organisation is an active conspiracy against democracy and seeks to work inside the State in order to subvert it. Where such an organisation has muscle and finance, there may be a case for a ban. The BNP, for the moment, is thankfully nowhere near such a point.

The defenders of liberty should remain willing to trust the power of their ideal.