What is the most popular political grouping in America right now? It's not the Democrats, who've long since come down from the Obama inauguration high of just 12 months ago. It's not the Republicans, for all their hopes of big gains in this November's mid-term elections. No, according to one respected poll, it's a ramshackle, hot-blooded, iconoclastic and thoroughly conservative movement called the Tea Party.

The inspiration is ancient: that fabled popular uprising against remote and tyrannical government that took place in Boston harbour in December 1773. But the history of the modern Tea Party is even shorter than the reign of King Barack. Its birth is generally dated to 19 February 2009, when an excitable market commentator for the CNBC business news cable network railed in a live broadcast from the floor of the Chicago Mercantile Exchange against the government's plan to bail out mortgage owners in over their heads.

The scheme, said Rick Santelli, simply "promoted bad behaviour" by "losers". Far better, he went on amid loud cheers from traders, to "reward people who carry the water, instead of drink the water... are you listening, President Obama?" If not, Santelli warned, "we're thinking of having a Chicago Tea Party". That was 11 months ago and, basically, the Tea Party people have been at it ever since.

They are of course detested and mocked by America's right-minded media and intellectual establishment. For Democrats, they make perfect targets, depicted as mindless Neanderthals egged on by ranting right-wing talk radio and TV hosts, deafened to the real world by the conservative echo chamber they inhabit. The Republican leadership, meanwhile, looks on them with a mixture of condescension and trepidation.

Predictably, criticism from the elites has only fired up this pitchfork army of the neglected middle class. Where Obama is cool and analytical, Tea Party-ers are noisy and emotional. Most important, there are millions of them, though the lack of a formal party structure makes it hard to estimate their numbers with precision.

But an NBC/Wall Street Journal survey in December found that, while only 28% of voters had a positive view of Republicans, and 35% of Democrats, fully 41% looked kindly on the Tea Party movement. Its policies are less platforms than visceral howls of opposition: to immigration, to deficits, to excessive taxation and excessive government and, loudest of all right now, to the healthcare reform bill the Democratic Congress looks set to send Obama for signature in a few weeks.

And next month sees the first Tea Party national convention, featuring as keynote speaker – who else? – Sarah Palin. Tellingly, the lady has chosen to pass on CPAC, the traditional national gathering of conservatives held in Washington each February, to attend the inaugural Tea Party gathering in Nashville.

What happens thereafter is perhaps today's most intriguing single question in American politics. In the short term, the Tea Party could mean big trouble for Republicans just as they were licking their lips at the prospect of big gains on Capitol Hill – an expectation only heightened further by the announcement by two prominent Democratic senators last week that they would not seek re-election.

Now though, the party faces new internal divisions that could be fatal. In a special Congressional election two months ago, right-wingers – Tea Party men in all but name – forced out the official Republican candidate, splitting the party and handing Democrats a seat they had not won since the Civil War. Much the same seems to be happening in Florida, where Tea Party opposition may have doomed the state's popular Republican governor Charlie Crist, a moderate, in his quest for an eminently winnable Senate seat. Now activists mutter darkly about giving the same treatment to Republican candidates in other states who stray from the true path.

The Tea Party is a movement, not a party. The fact that it doesn't have a leader probably enhances its popularity. In so unstructured a movement, everybody can find something to their liking. Quite possibly, it will become the right-wing equivalent of MoveOn.org, the liberal ginger group that was born of protest against the Clinton impeachment in 1998.

Every sign is this winter of discontent in the US will last a while yet. The polls may overstate the Tea Party's popularity, but the malaise and frustration that fuels the movement will not soon go away. One way and another, next month in Nashville could be fun.