Why the defection of a little-known MP is so revealing about the state of British politics

Another heavy defeat for the Tories will provoke the civil war they should have held after their trouncing in 1997

The defection of a relatively unknown Conservative MP is much more significant than it seems. Defections are the most reliable guide to the state of British politics, much more so than opinion polls and the results of by-elections. British politics is tribal. Active members of a party tend to fight their cause from within. They support their party like others follow a football team - through good times and bad. When MPs defect they light up the political stage, revealing much about the party they leave behind and the one they have joined.

The defection of a relatively unknown Conservative MP is much more significant than it seems. Defections are the most reliable guide to the state of British politics, much more so than opinion polls and the results of by-elections. British politics is tribal. Active members of a party tend to fight their cause from within. They support their party like others follow a football team - through good times and bad. When MPs defect they light up the political stage, revealing much about the party they leave behind and the one they have joined.

In the 1970s and 1980s, Labour occasionally won by-elections and often commanded leads in opinion polls. Yet all the high-profile defections were away from Labour: either to the Conservatives or the newly formed SDP. The defections demonstrated, more accurately than mid-term polls and elections, that Labour was still a long way from power. The defectors switched sides for reasons of ambition, principle or both. They sniffed the prevailing winds and concluded that they needed to move on. In doing so they were living proof that Labour had not changed enough to win an election.

From the mid 1990s, the movement has been almost entirely in the other direction, with senior Tories switching to other parties quite often leaping over the Liberal Democrats and joining Labour. Robert Jackson's defection at the weekend demonstrates vividly that the Conservatives languish even after two election defeats. At the equivalent point for Labour in the 1980s the defections had stopped. Its political recovery was underway.

The bleak symbolism of Mr Jackson's switch provided the context for yesterday's launch of the Tory plans for tax and spending, the most important event for the party since the last election. At the launch, Michael Howard emphasised his support for choice, reforms and investment in public services, paid for by cutting out waste.

Part of Mr Howard's problem is that Mr Jackson's new leader has already outlined the same objectives. If Messrs Blair and Brown had not in their different ways got there first, I wonder whether the Conservatives would have sought to trump them in terms of the amount they plan to save. There are already doubts about whether Mr Blair and Mr Brown will be able to implement their proposed savings of £23bn. Mr Howard has said he will cut costs by £35bn.

Since the party conference last October, Mr Howard has been busy celebrating the modesty of his proposals. Indeed, the emphasis on cautious restraint has been at the heart of his appeal. He has insisted that, in order to win the trust of the electorate, he would promise only what he could deliver, making the most of apparently achievable incremental measures. Yet, in spite of himself, Michael Howard is promising a revolution.

There will be £35bn of spending cuts and, on other policy fronts, a conflagration in Europe as he renegotiates the basis of Britain's membership and an unprecedented expansion of the private sector in health and education. All this is planned for the first week of a Conservative government, or almost.

The more detailed messages follow a similar pattern. Yes there will be tax cuts, but not as many as they would like because of spending commitments and the need to balance the books. So although there is a revolution it will not be as revolutionary as they had hoped, because they are being highly cautious.

Mr Howard is a serious politician who frets admirably about the practicalities of policies. His independent review of possible savings in public spending is a much more substantial piece of work than the flimsy preparations that preceded William Hague's Tax Guarantee in the run up to the 2001 election. But Mr Howard ends up in a similar position as Mr Hague, promising increased spending on public services and tax cuts.

At the very least, there is a problem relating to the sequence in which these latest proposed savings will be implemented. Yesterday the Shadow Chancellor, Oliver Letwin, pledged to cut taxes in his first budget, within a month of a Conservative government. But some of the savings to pay for the tax cuts will take years to bring about - if they can be brought about at all.

The dividing lines between Labour and the Conservatives in the election campaign will therefore not be greatly different from the 2001 election. Labour will dare to argue against the tax cuts being proposed by the Conservatives and in favour of its own spending commitments; but it will be too frightened to put the case openly for any tax rises.

This is progress of sorts. In the 1980s and early 1990s it seemed that the promise of tax cuts was in itself a guarantee of election victories, irrespective of the consequences for Britain's declining public services. Now at least a party has a chance of winning an election by warning that tax cuts threaten the improvements in public services.

It still leaves the debate about taxation and public spending in Britain at a depressingly unrealistic level. In effect, voters are still being told that they can expect European-style public services while paying US levels of taxation. Tax as an issue remains almost as much of a taboo as it was in 1997.

So Mr Jackson can switch to Labour knowing he will be untroubled by new policies relating to higher taxes, while hailing Mr Blair's leadership in the war against Iraq and his introduction to top-up fees for universities. No wonder the Conservative leadership is confused. One of the most intelligent frontbenchers told me yesterday: We lost Robert Jackson to Labour partly because we were opposed to a market in universities and the introduction of fees for students. Yet we got no credit with the voters for dropping our previous strongly held support for markets and fees for students. Until we disentangle this political knot we are buggered.

The centre-right of Mr Blair's big tent is firmly protected. Few will be leaving the tent to support the Conservatives and, like Mr Jackson, some are still moving in from the right. At least one Conservative MP is contemplating defection after the election. Meanwhile, other senior Tories are already planning their leadership bids.

Neurotically fearful of a Conservative recovery, Mr Blair has given Mr Howard little political space on domestic and international issues. But the exit at the opposite end of Mr Blair's big tent is less secure, as some on the centre-left contemplate a move to the Liberal Democrats. No wonder Mr Kennedy contrived his own launch yesterday, describing his party as the real opposition. Yet the Liberal Democrats are still an opposition party that cannot win a general election. The Tories are in no position to win either. That leaves a single victor.

There will be much turbulence in the Labour party after the election, not least at the top. But it is less traumatic carrying out internal battles in the context of an election victory. For the Conservatives, another heavy defeat will provoke an outbreak of the civil war they should have held and resolved after their trouncing in 1997.

s.richards@independent.co.uk