They are straining to take over and tear up the Bush legacy, such as it is, by the roots. If the elections give Democrats the presidency and increased majorities in both houses, as seems likely, the US is going to see one of the most radical alterations in its political outlook for decades.

As things stand right now, the politics is all good for the Democrats and all bad for the Republicans. The time-series of national opinion-poll ratings for the Republican presidential candidates looks like the read-out of a patient having a stroke. The lines jerk up and down, as party supporters search desperately, and so far in vain, for a candidate they like. The surge from nowhere of Mike Huckabee – to join a three-way tie with Rudolph Giuliani and Mitt Romney – is partly a sign of this desperation. It threatens to split and even destroy the Republican coalition, by dividing social conservatives from economic conservatives.

Mainly, though, it underlines something even more significant than that: the growing appeal of economic populism among supporters of both parties.

Mr Huckabee is an evangelical – his faith leads him to reject the theory of evolution and to favour constitutional amendments to ban abortion and gay marriage – but he is an economic populist as well. On trade, on the tyrannical power of Wall Street and sometimes even on public spending, he sounds more like John Edwards than Mr Romney. Trade must be “fair” not free. There must be an end to “shipping jobs overseas”. He rails against outlandish CEO pay. He is not averse to more regulation, or to spending (financed with higher taxes) on many social programmes. Immigration complicates the picture: as governor of Arkansas, his treatment of illegal immigrants was too soft for many Republicans. But he is making up for it now, promising to fence the border and deport (and bar from re-entry), immigrants who fail to register within 120 days.

“The Wall Street to Washington axis, this corridor of power, is absolutely, frantically against me,” he boasted this week. To be sure, for many Republicans, his main attraction is his religious conviction; he is also a likeable, funny, easy-going man and good on television. Nonetheless, the economic populism is all upfront and has plainly struck a chord.

The Democrats, of course, are way ahead of him. Also conflicted on immigration – the party wants to appeal to Hispanic voters without angering the unions and the white working-class – they are going strong on fear of trade and on rage at corporate greed. And there is no mystery about why. Most clearly in the case of trade, they are reflecting public sentiment.

A recent poll by the Wall Street Journal and NBC found that 58 per cent of Americans think that globalisation has been bad for the US and that only 28 per cent believe that it has been good. Ten years ago the split was more even: 48 per cent thought that globalisation was good and 42 per cent that it was bad. The biggest surprise is that supporters of the two parties are no longer far apart on the question. Globalisation has been bad for the US according to 55 per cent of Republicans and 63 per cent of Democrats.

Political stasis is likely in 2008 in any case, with an isolated and unloved president on one side and a rabid uncompromising Congress on the other (to say nothing of the distractions of the campaign). But the rising tide of economic populism makes the prospects for meaningful trade liberalisation and an intelligent resolution of the immigration conundrum even more bleak. Beyond next year, the real question is what happens in 2009, when (let us suppose) the main obstacles to a populist turn in policy have been removed. How far left, in economic policy, might America then veer?

The answer is a long way. As a rule, the stridency expressed in the primaries fades once the nominees are chosen and the general election approaches. Debate gets tugged back to the centre as candidates try harder to appeal to independents. That cyclical correction will happen again next year, but it will be milder than usual because populist economics is selling so well across the board.

It is striking, too, that the growth of anti-trade and anti-corporate sentiment has happened up to now in a relatively benign economic environment. Stagnation in middle-class living standards, erosion of manufacturing employment, financial stress, alarm over the cost of health insurance and a college education – all these concerns came to the fore with interest rates, inflation and unemployment low, with the stock market strong, and with aggregate output (until very recently) growing at a reasonable pace.

Add an election-year slowdown, already begun or even an outright recession to this mix. Add rising unemployment. Add an accelerating fall in house prices and a gathering wave (counter-measures notwithstanding) of mortgage foreclosures. If populism had material to work with in 2007, just wait for 2008.