The Big Question: What is the Tea Party movement, and could it change US politics?

Why are we asking this now?

Scott Brown, the shock Republican winner of this week's election in Massachusetts to choose a successor to the late Senator Edward Kennedy, is America's latest political sensation. But the most significant recent development in US politics is the emergence of the Tea Party movement, a populist organisation that contributed to Brown's victory, and which could reshape the country's political landscape at November's mid-term elections here.

How did the movement begin?

The name, obviously, derives from that most celebrated anti-government insurrection in American history, the Boston Tea Party of December 1773. But its re-emergence in the early 21st century is usually dated to the outburst of Rick Santelli, a commentator for the CNBC business news cable network, from the floor of the Chicago Mercantile Exchange a year ago this week. Railing at the new administration's plan to bail out mortgage owners who had taken out loans they couldn't afford, Santelli declared that the scheme "promoted bad behaviour by losers". Was President Obama listening, he asked. If not, "we're thinking of having a Chicago Tea Party".

Then what happened?

The name caught on, and Tea Party groups took root across the country. In April 2009, Tea Party-ers staged anti-tax protests across the country. During the summer they held rallies that often drew thousands of participants, aimed in particular at the Obama healthcare plan, the emblem of all they revile. The climax was a demonstration on the Mall in Washington on 12 September, described by supporters as the biggest conservative protest in modern American history.

So they are conservatives?

Basically yes, though of a very fundamentalist and angry variety. They are defined less by what they are for, than what they oppose: runaway government spending, high taxation, and large deficits, epitomised by Obama's healthcare reform and the $787bn stimulus package in February. It is also a cry of fury by the average Joe – ordinary Americans suspicious of pampered elites, and disgusted by bailouts of the undeserving. These range from those who stupidly buy homes they can't afford to greedy Wall Street banks and incompetent, eternally loss-making car companies. As Santelli said on February 19 last year, far better "to reward the people who carry the water, rather than drink the water". Tea Party-ers also oppose immigration. But there's a powerful libertarian, anti-establishment streak in the movement as well. In that sense it appeals to independents, who refuse to align themselves with either established major party.

How big is the movement?

Since it's a movement and not a structured party, it's hard to say. But by any loose definition there must be millions of them. Indeed, an NBC/Wall Street Journal poll two months ago suggested the Tea Party might be the most popular political organisation in the land. Over 40 per cent of voters had a positive view of it, compared with 35 per cent for Democrats, and 28 per cent for Republicans. The picture will be clearer after the first Tea Party national convention in early February in Nashville, Tennessee.

Who is its leader?

It really doesn't have one, although the keynote speaker in Nashville will be Sarah Palin, the 2008 Republican vice-presidential nominee (for which she is reportedly being paid $100,000). Its activities are closely chronicled by the conservative Fox News cable network. Conservative talk radio provides a powerful echo chamber, not least via the vitriolic Rush Limbaugh. A few Republican right-wingers in Congress are identified with it. But there's no well-known specific national spokesman for the movement.

But it surely benefits the Republicans?

For the time being, absolutely. More than anything, the Tea Party is "anti-Washington" – and Washington right now is run by Democrats. Republican leaders reckon they can ride Tea Party energy and anger to big gains in November's mid-term elections, which could hand control of at least one chamber on Capitol Hill back to Republicans. After Massachusetts, Democrats are petrified that this grass-roots fury could topple dozens of their Congressional incumbents. In a message this week, the "Tea Party Express" – the closest thing the movement has to a national structure – issued an ominous warning. "To Barack Obama, Nancy Pelosi and Harry Reid [the last two the Democratic leaders in the House and Senate]: We here at the Tea Party Express want you to know that this is just the start. We'll see you and the candidates you back on the campaign trail."

But could Republicans be hurt?

Wiser heads in the party are alive to the danger. Right now, as one senior Republican Senator puts it, there is a "convergence of thought" between Republicans and the grass-roots activists "that will be a catalyst to send more conservatives to Congress". The danger, though, is that the Tea Party will push Republicans so far to the right that it will become unelectable. In a closely watched special election last November, Tea Party and other right-wing activists effectively split the Republican vote, handing Democrats a Congressional seat in upstate New York they hadn't won since the Civil War.

In Florida, Tea Party hostility could doom the quest of Charlie Crist, the state's popular governor, for an eminently winnable Senate seat this autumn. The risk is that across the country, moderate Republicans like Crist could face primary challenges from conservatives, pushing the party further right. And if, and when, Republicans do win power again, they will be in the incumbents' hot seat.

What happens now?

All eyes are on Nashville. If the Tea Party turns itself into an organised party, the omens are not good. Under America's winner-take-all electoral system, third parties do not fare well in national politics. Teddy Roosevelt took that course in 1912, and was defeated. Ross Perot, the most important third-party White House candidate of recent times, failed to carry a single state in 1992 – although he won a fifth of the popular vote riding a wave of resentment not dissimilar to today. In 2000, Ralph Nader's Green Party was a spoiler. Though it probably cost Al Gore the election that year, the Greens amassed only 2 per cent of the vote.

So the Tea Party will remain a movement?

That seems the most likely outcome. That way, it can portray itself as above the sordid political fray in Washington. Its lack of a detailed policy agenda will, if anything, broaden its appeal, while the establishment of a leader and an internal bureaucracy might create the impression that it is just another party – as corrupt, selfish and petty-minded as those that are so grievously failing the country now. Possibly, the Tea Party will end up like MoveOn.org, borne of left-wing anger at the impeachment of Bill Clinton in 1998, but which is now a liberal ginger group – influential and important, but which does not run its own candidates at elections.

So will the movement last?

Yes...

* Once the Tea Party declares its true colours, voters will reject it

* Sooner or later it will be co-opted into the Republican party

* To survive in US politics, you need money, which the Tea Party lacks

No...

* American voters are unprecedentedly angry and thus open to outsiders

* The two major parties have lost touch with the national mood

* Populist movements have a long history in US politics