The British National Party and the UK Independence Party are the two elephants in the room for the Labour and Conservative parties. Neither will ever form a government, but from time to time they wallow in their 15 minutes of electoral fame, forcing the main parties into reluctantly commenting on subjects where silence is the preferred political response. The BNP and UKIP have the capacity to shock and, since the establishment of the Lib Dems as a serious third-party contender, they are now the outward and instant methods of choice for voter disaffection.

Protest votes are often cast in the heat of the moment, with little thought given to the longer-term consequences. The BNP will not make a breakthrough next month (there are only 360 BNP candidates out of 4,000 contests), but a dozen BNP mid-term election gains from Labour in each of only half a dozen councils are far more likely to shake the body politic than two or three hundred council-seat gains by either the Lib Dems or the Tories.

In the days when they could squeeze into a taxi, the Liberal Democrats were a party of protest. Voters would use the opportunities presented at by- elections and local elections to vent their frustrations by giving their vote to the third party. A vote for the Lib Dems now, however, is no longer a vote for protest, it is a vote for a party with serious prospects of power - at least in local government. The Lib Dems have consequently become part of the political establishment at a time when voter disaffection is thought more likely to be addressed by anti-establishment parties.

So just as the Lib Dems have vacated the role of the dustbin of politics, the vacuum is filled by the BNP and UKIP. Not for one moment am I suggesting that the Lib Dems have ever sought to attract, on the basis of fear, the type of voters currently tempted to vote BNP. But it is a simple statement of fact that as parties move nearer to power, other new parties will seek to address - and exploit - the fears of the alienated and dispossessed. There has always been a protest vote at elections other than general elections (the Greens in 1989 and UKIP in 2004), and Margaret Hodge clearly thinks that the 2006 local government elections in east London could be the year of the BNP.

Whether Ms Hodge was right to air her fears publicly is an open question. She may be tactically talking up the threat in order to increase Labour turnout. But Labour apparachiks were clearly scared by her public ruminations - so much so that she appears to be rowing back from her original comments in the weekend press.

Some will argue that she has merely given the BNP a massive shot of the oxygen of publicity that their own resources could not achieve. But silence from the main parties on voter alienation caused by the impact of sudden demographic change, is probably part of the reason for the growing BNP support in Barking and Dagenham, where traditional Labour voters feel that no one is listening to them. And with the overwhelming evidence adduced by the Rowntree Trust of growing potential for BNP support, along with the research conducted recently by the Spectator's Peter Oborne, it would seem only sensible for at least one government minister to speak out publicly about her fears.

Mr Oborne recently observed the BNP canvassing in Ms Hodge's constituency, and found that potential BNP voters were invariably respectable working-class or even lower-middle-class voters - and they could give the BNP enough seats to become the main opposition party. Whether we like it or not, racist politics may yet be on the march.

This is thanks to the decision of the two main parties effectively to discount or ignore all voters who are not the 800,000 electors in the target 100 marginal constituencies. Oborne notes that the neighbouring Dagenham Labour MP, John Cruddas, blames New Labour's obsession with the preferences and prejudices of the swing voter in the swing seats for "driving the white Dagenham working and lower middle classes straight into the arms of the BNP".

The targeting of such a defined voter profile excludes general policies tailored to the working-class masses. For the Tories, the same game is being played, and there was a hint of criticism in yesterday's interview on Today by Iain Duncan Smith - who represents neighbouring Chingford - that the Tory party leadership's obsession, also, with only swing voters has caused the Tories to vacate the field in Labour areas of disaffection, leaving the pitch clear for the BNP.

The Tory party's problems in handling the BNP are compounded by their recent experiences in the last general election. Michael Howard raised the issue of race and immigration to the exclusion of the rest of his agenda. The swing voters reacted badly, so now anything to do with race, asylum and immigration is, like tax and Europe, off the Tory political agenda. To get a front-bench Tory to muse on the current debate is impossible.

"The Conservatives refused to comment" was invariably the last sentence of every newspaper article of the past few days since Ms Hodge gave her interview. But as Ann Widdecombe noted yesterday, race and immigration have often been the sleeper issues on the doorstep.

Three related issues stand out for the main parties to ameliorate voters' concerns: health, housing and education. It seems a patently natural response for voters in areas of sudden demographic change to get hot under the collar if they now have to wait 10 days for a GP's appointment, if they are denied their previous place on the social housing list, and if the character of the classroom is so changed that within two years English is no longer the first language.

I doubt that white Dagenham and Barking voters are any more racist than their counterparts elsewhere in London where integration has over time proved successful. But it is inevitably the case that, if local services are completely overwhelmed, there will be resentment at the ballot box where the shoe pinches hardest.

The worst response of all is to attack the voters. Ritual denunciations of the BNP the morning after the night before are usually the best recruiting ground for a further BNP advance in the future. If normally loyal Labour voters are turning to the BNP along with former Tories (because sometimes there are not even Tory candidates), there is a message to the main parties.

Blaming voters for doing the wrong thing is not usually the best way to win them back to the mainstream. Listening to Ms Hodge and Mr Cruddas might be a good starting point - provided they are not silenced by the rest of the political establishment.

mrbrown@talktalk.net