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But each insult and attempt to marginalise them seems only to have stiffened their resolve and swelled their numbers. Polling indicates that they are now more popular than either Republicans or Democrats. Despite all the claims they are extremists, around half of the electorate now identifies with the Tea Party and up to a quarter view themselves as members.

The time for them to be taken seriously is long overdue. The Tea Party will be pivotal in November. It has ripped up the playbook for the 2012 Republican primaries. Asnd it could just end up kicking President Barack Obama out of the White House.

Christine O'Donnell's Senate primary victory in tiny Delaware last week rocked the world of the Republican establishment in Washington. Widely portrayed as a kook and bitterly opposed by the Republican party, she defeated Mike Castle, a moderate Republican who had held continuous political office in the state since 1965.

Castle was widely regarded as a shoo-in against the Democrat on November 2nd. O'Donnell is a political novice and a patchy campaigner. The secret of her win? Endorsements by Sarah Palin, the former Alaska governor, and the Tea Party.

Oh, and Delaware primary voters didn't give a fig about Castle's electability. As far as they were concerned, he was a professional politician who was a RINO - Republican In Name Only - who could not be trusted. In the Senate, he might switch parties or vote with the Democrats to give Obama a bipartisan sheen. And, after 45 years in office, why not give someone else a chance?

The Tea Party has often been portrayed as a fringe group within the Republican party or a support network for Palin.

Dig deep and you will find that it is neither. There is, of course, a problem with the word "it". Although the Tea Party is referred to as a single entity, it is really a loose conglomeration of some national organisation and a plethora of groups formed in local neighbourhoods all over America. Standing for Taxed Enough Already, it took its inspiration from the 1773 Boston Tea Party protest against British colonialists.

A desire for small government, lower taxes and fidelity to the United States Constitution binds members together. There is a prevailing mood of anger towards Washington and a sense of having been conned. Mention the $700 billion (£448 billion) bailout, health care reform or cap and trade legislation and they will go into conniptions.

But beyond that, the Tea Party is a vast, teeming muddle of opinion and impulses. Many of its strong supporters don't attend public meetings. "The Tea Party is more an attitude than anything organised," one Southern conservative told me.

I used to think that the highest expression of American democracy was the Iowa caucuses. There, on a freezing evening in Des Moines, Iowa in January 2008 I watched ordinary citizens trudge through the snow to pick their favourite Democratic presidential candidate.

Obama was their man and the caucuses were his launch pad for the White House. Checking back, I described the scene in the school gym as "grassroots politics at its most vibrant". "