One of the more interesting questions in British politics is not why the BNP has performed so well in recent elections, but why it has performed so poorly. The MPs' expenses scandal and a wild recession form a dream combination for a bunch of fascist outsiders. Yet in the two seats the BNP won during last summer's European elections it secured fewer votes than four years earlier.

Nick Griffin and his party have no excuses. There are plenty of other ingredients in place for a far-right party to thrive fleetingly. The mainstream parties are in serious decline and so is the culture of orthodox party politics.

Several senior politicians from Gordon Brown downwards, or upwards, tell me they can attract big crowds at a book festival or at a meeting of Make Poverty History, whereas if they deliver the same speech at a party meeting only a handful of people turn up. Party membership is falling and funding remains a nightmare. The decline of the bigger parties leaves a potentially dangerous gap in the market and one that the likes of Esther Rantzen and assorted independent candidates will not fill on their own.

In particular the expenses' saga is a gift wrapped crisis for the BNP. At a time when voters feel insecure their disillusionment with elected politicians is seemingly vindicated. Parts of the media play their role too in the decline of mainstream parties, implying that virtually all politicians are crooks and liars. Their treatment of the last three Prime Ministers should have been a perfect recruiting agent for the BNP.

Messrs Major, Blair and Brown have drowned in a sea of allegations about sleaze and questions in relation to their integrity. Yet when historians review the trio will many conclude that by some weird coincidence all three were corrupt? I doubt it.

Raging angrily on this fertile terrain the BNP has been hopeless. It won a total of 6 per cent of the vote in last summer's European elections, the height of the recession and the hysteria over MPs' expenses. As the pollster, Peter Kellner, pointed out at the time only one elector in three turned out to vote, which means just 2 per cent of the total electorate voted BNP. Research carried out by YouGov found that roughly half of BNP's voters were truly racist, the other half were people who feel insecure and alienated from the main parties. In other words just one per cent of the electorate last summer were racist BNP voters.

The research suggested that other voters were angry about immigration and related issues and could be swayed but had not been so far. Kellner argued rightly that the real challenge was to deny the BNP the exposure and credibility it needed to detoxify the brand.

Cue the BBC's mistaken decision to give Griffin a platform on Question Time tonight. As I have argued before the BBC had a duty to give the BNP airtime. It was not the BBC that voted for the BNP at the last European election. The voters have to answer for that one. But it was the BBC which decided to give Griffin his ideal platform, a highly charged audience, lots of questions, and a big panel making forensic scrutiny impossible.

Griffin's newsletter to his supporters yesterday highlights why the BBC was wrong. He describes tonight's Question Time as a "milestone in the indomitable march of the BNP towards saving our country". Showing an awareness of how the programme might play out he launched a pre-emptive strike: "I will no doubt be interrupted, shouted down, put on the spot... It will be political blood sport".

Griffin cannot lose. If he is given a hard time he will be the programme's noble martyr. If he has an easy ride he will enjoy the journey. I can hear him now in dialogue with the conveniently large panel and hyper active audience:

"Unlike Jack Straw I don't just call for British jobs for British workers. I believe it....unlike Baroness Warsi (the Tory panellist) we will offer a referendum on the Lisbon Treaty even if all the countries have ratified it... we mean what we say and at a time when we are fighting illegal wars and have politicians with their fingers in the till it's about time a party spoke up honestly for the hard working decent British people".

David Dimbleby: Right I want to bring in the audience at this point.....

Audience: Anger... Cheers... Jeers (all flattering to Griffin who is centre of attention throughout).

Last month James Macintyre, a former producer on Question Time, revealed in the New Statesman that the programme's makers wanted Griffin on the panel two years ago, long before the BNP's limited breakthrough in the summer. To its credit the BBC resisted at first the naïve showbiz instincts of some who run the independent production company that is responsible for Question Time. According to Macintyre the company, Mentorn, persisted for two years and finally got the go ahead from the BBC. This suggests that Mentorn wanted what Peter Hain calls a "beanfest" for reasons well removed from the BBC's charter obligations.

The BBC's obligations to the BNP only became an issue after the European elections. They could have been addressed without giving Question Time and the BNP a joint box office hit, the programme makers and Griffin dancing together as ratings soar.

My guess is that the dance will not last for very long, at least as far as Griffin is concerned. Probably he will perform well tonight in an ideal media setting for his slippery charm. But he and his party are useless. In his row with various army leaders this week he referred to Sir Mike Jackson as Ken Jackson, the former union leader. That was in a written statement which he had a chance to check. He is incompetent, always a big flaw with fanatics.

A party fuelled by hatred will also at some point turn in on itself. This is what happened to the National Front in the 1970s, an organisation that the BBC was careful not to allow on light entertainment current affairs programmes, as Question Time has become. Its leading figures were so full of bile they started to hate each other. Soon there were splits and divisions within the splits. Probably this will happen to the BNP.

That is my hunch, but I might be wrong. These are unusually febrile times. I speak to a lot of MPs who worry about the impact of the BNP in their constituencies more than virtually any other issue. A Cabinet minister also said to me recently that the simultaneous political and economic crises are bound to have tumultuous consequences, so far ill defined. There is still a dangerous gap in the market. The BNP show few signs of filling it, but now an opportunity has arisen from nowhere for its leader to perform.

Those who watch uneasily tonight should remember that in the end the only potential beneficiary of the relentless attacks on democratic politics as a vocation is a party that loathes democracy. Perhaps it is time for the MPs' expenses saga to be put in some sort of context.

At the same time the programme's makers, who had a ball with the MPs' expenses, should recognise that there is enough dark comedy and high drama in politics at the moment without the need for them to contrive dangerous controversy. The BNP is useless, but it does not deserve a helping hand.

s.richards@independent.co.uk