Fans of football teams with no hope of winning the league get their thrills from derbies or the occasional giant killing in the early rounds of the FA Cup. In politics, the top prizes are sewn up even more tidily than the Premier League. At Westminster, Labour and the Tories fight over first place, while the Liberal Democrats creatively explore different ways to come third.

A new season is about to get under way and, while no one can rule out a spectacular upset, it looks as if Team Cameron has the title in the bag.

Scotland and Wales have their own nationalist dynamics, of course. But in England, too, there is action away from the centre that, like a good mid-table clash, makes for compelling viewing, even though less is ostensibly at stake. The prize is fourth place and two parties - the Greens and the BNP - are in contention.

In London's mayoral election, the Green candidate Sian Berry and the BNP's Richard Barnbrook received 77,374 and 69,710 votes respectively. The Greens also kept the BNP in fifth place in the ballot for the London Assembly, taking two seats to the BNP's one. Meanwhile, the far right beat the environmentalists in four out of 14 Assembly constituencies. Nationally, at local council level, the Greens are just ahead of the BNP - 47 seats to 37 - though the far right made twice as many gains as the Greens in May's elections. In June's Henley byelection, 73 votes separated the Greens and the BNP in third and fourth places. Labour came fifth.

So what? First past the post makes it impossible for tiny parties to break through and, with around 1 per cent of the national vote each, these minnows would struggle under any voting system. Westminster is not about to be overrun by hippies or fascists.

But the Green versus BNP competition matters. For a start, if a party dedicated to saving the 'traditional British genotype' from 'extinction' by mixed-race marriages gets roughly as many votes as a party dedicated to saving the planet from climate change, something is wrong.

Of course, the Greens have had their issue poached by the big three, while the far right has anti-miscegenation policy all to itself. But fringe parties don't compete with the mainstream in the usual way. If voters are simply shopping around for a party that sounds tough on immigration, for example, they could vote Tory or, for that matter, Labour, which has tacked steadily rightwards on the issue in recent years. The point of the BNP is that they see everything - housing, the economy, crime - through the prism of race. When mainstream politicians talk about immigration as a problem, the potential BNP voter takes it as confirmation of his fears, while retaining the suspicion that the Establishment won't do anything about it. Green voters feel similarly about their cause. Labour and Tory pledges on the environment are seen as empty rhetoric. (The crucial difference is that the Greens' cause is a good one.)

A fringe party represents people who meet two criteria. First, they deeply distrust the three main players, seeing them as out of touch, dishonest and incapable of addressing some issue that matters to them above all others. Second, they bother to vote. Think of the Greens and the BNP competing to lead the political wing of British cynicism. Clearly, that is another reason why we should care which of them does better.

That cynicism thrives when people feel alienated from a distant ruling class, believing that, for all their professed differences, the main parties are basically 'all the same'. That is not a difficult proposition to sell at the moment.

The great truism of English politics is that elections are won on the centre ground. Tony Blair held it for 10 years, now David Cameron claims it. A big argument these days amounts to the whether there should be more academy schools or loads more academy schools? Should the NHS be opened up to 'competition' or just 'contestability'?

And that is the concrete stuff. In the absence of competing policies, a bizarre battle has erupted for possession of abstract nouns. Immigration Minister Liam Byrne, often dubbed a 'rising star' of the Labour party (can stars rise when the sky is falling?), wrote an article last week rubbishing David Cameron's claim to the idea of 'fraternity'. The Tories, meanwhile, in a bid to undermine a Gordon Brown comeback in the autumn, are campaigning this week on the issue of 'fairness', the Prime Minister's new favourite word after he lost 'prudence' to the credit crunch.

All this posturing is based on evidence that the voting public, as it showed by rejecting the Tories in their angry, sulking years, likes moderation. But that doesn't tell you anything about the non-voting public. More people abstained in both of the last two general elections than endorsed the winning side.

That might be because we are a nation of sloths who only bother to choose between candidates if they are up for eviction via premium-rate phone line. But it is also because political debate has become so arcane and is getting worse. Is David Miliband Blairite, post-Blairite or Blair-lite? To people with better things to do than read think-tank pamphlets all day, the difference is not obvious.

All parties are in the centre, but with no clear direction of travel. Their political compasses are liable to be influenced by whichever fringe manages to exert the strongest magnetic pull. That gives influence to a maverick fourth party, especially when one of the big three is lost in the wilderness. When the Tories were in disarray, they were further destabilised by the UK Independence party. The presence of a militant anti-Europe wing over the past decade successfully skewed mainstream politics. England now has a fixed Eurosceptic bias.

Ukip looks like a spent force. It didn't achieve withdrawal from the EU, but helped to shape the debate. The same process could affect the way mainstream parties talk about the environment or immigration, depending on how the Greens and the BNP perform.

The BNP's recent local election results were surprisingly good given that the party is in schism. Last year, a 'Real BNP' faction formed in rebellion against party leader Nick Griffin, who is accused of failing to modernise, being 'arrogant' (just like in ordinary politics) and deploying the 'security department' of the party to burgle rebel members' homes (not so like ordinary politics). Richard Barnbrook, who, as a London Assembly member, is the BNP's highest elected official, has a different challenge building a substantial far right following. He has to live down HMS Discovery: A Love Story, a film he made in 1989 that includes scenes of naked men frolicking and a soundtrack of apparently gay erotic poetry. 'It was an art film, end of story. Not a bloody porn film,' he says.

For the Greens, the problem has not been too many leaders, but none at all. They have traditionally been led by two 'principal speakers' mandated to represent the party in public, but not decide policy. That will change next month when they hold their first leadership election, which is almost certain to be won by MEP Caroline Lucas. She is intelligent and articulate. Griffin just about manages louche charisma.

The question is: which of them would we rather see hogging the airwaves on election night? Both parties are hoping to gain their first Westminster seat at the next election - the Greens in Brighton Pavilion, the BNP in the new east London constituency of Dagenham and Rainham. Both need the votes of disgruntled Labour supporters to win. If, as looks likely, the Tories are cruising towards an easy victory, disproportionate attention will be paid to the impact of wild-card candidates. (Remember a foaming Jimmy Goldsmith taunting David Mellor out of his seat in 1997?) As Labour heads into the wilderness, there will be much opining on what the protest vote meant.

Everyone takes some interest in who tops the political Premier League. After all, the winner gets to run the country. But there are rewards available lower down the table. There is a nail-biting competition for the Uefa Cup place of English politics - the opportunity to give political shape to much wider scepticism and disillusionment; to be the flag-bearer for anti-Establishment feeling. Our nation would be in a sorry state if the BNP beat the Greens to that prize.

· Andrew Rawnsley is away.

· This article was amended on Monday August 18 2008. In London's mayoral election, the Green candidate Sian Berry did not comfortably beat the BNP's Richard Barnbrook by 409,101 votes to 198,319, as suggested in the article above. The candidates received 77,374 and 69,710 votes respectively. This has been corrected.