There is only one winner if the Prime Minister is defeated tonight

It's no disrespect to the great importance of the Hutton report to say that tonight, and not tomorrow, is likely to be the climactic moment of the second Blair term, perhaps of the Blair premiership.

History isn't everything; but the defeat on second reading of any government bill, let alone one to which the Prime Minister has repeatedly attached the highest importance, is a very rare event indeed. There have only been two cases since the Second World War and on each occasion there were extenuating circumstances which make them far less momentous than a defeat would be this evening.

The first was in 1977 when the minority Labour government lost the Redundancy Payments Bill by one vote because no pair had been arranged for the absent Prime Minister James Callaghan; the Bill was reintroduced in a modified form. And while the Thatcher government, like Blair's today, had a very healthy treble figure majority when it lost the Shops Bill on Sunday trading in 1986, the Bill was anything but central to the government's programme. What's more backbenchers were told in advance that because of a religious and conscience dimension to the Bill they would have a free vote on future amendments. My colleague Michael Brown, then a Tory MP, recalls the 1922 Committee meeting which was told on the eve of the second reading vote that the three line whip was merely an "instruction to attend" and that MPs who voted against the government wouldn't be disciplined.

By contrast, the stakes tonight could not be higher. The scale of Labour's majority, the centrality of the bill to the government's programme of public service reform, the evidence that a sizeable section of the rebellion are using it to remove Blair from office, and the fact that a defeat would leave him to face the consequences of Hutton as a severely wounded premier, conspire to make them so. That's one reason why he would almost certainly have to call a confidence vote this week in the wake of defeat as the one means of reassembling some of his shattered authority.

When John Major called a similar confidence vote in July 1993 after his government was defeated on its plan to exempt the UK from the social chapter of the Maastricht Treaty, he successfully tied to it a reversal of the previous day's vote on policy. That course is hardly open to Blair. In Major's case he had been defeated over a single clause of a bill, one with which the rebels actually agreed. This would be a defeat for the Bill itself; the most Blair could risk - and he may well do this if the worst happens tonight - would be to include in the confidence vote a general reiteration of the government's university fees policy. But even a likely victory on such a confidence vote might not stop him from being so damaged that you would have to wonder for how many months he would be prepared to remain in office.

Many of the hardcore rebels, of course, view this prospect with equanimity. Some who object to the policy itself see no reason why Blair's fate should be tied to it, and would be entirely happy, rather than merely obliged, to support him on a confidence vote. Those who simply want Blair replaced with Gordon Brown would equally happily regard a defeat for the government tonight as a means to that eventual end. But while the latter group probably have a clearer-sighted analysis of the consequences for Blair personally, both are missing something much bigger: the consequences for the future of the Labour government itself.

The blithe notion that Blair can be defeated tonight without putting that future at real risk is as hollow as it is still, to some, seductive. For entirely opportunistic reasons, the leader of the revived Conservative Party - which just happens to be five points ahead of Labour in a recent poll - has chosen to oppose the government without presenting any credible alternative. Those Labour MPs planning to join him in the division lobbies tonight, would do well to understand the results of their success tonight.

Mr Howard has all the agility and ruthlessness needed to convert the very victory he has been essential to procuring into what may well prove a decisive advantage for his own party. Any Labour MP who imagines that Mr Howard will not be lethally swift in depicting the party as having reverted to its unelectable old Labour past, had better think again. Some Labour MPs worry that a Tory commitment to abolish all fees will be troublesome at the next election if the government succeeds tonight; in fact the Tories will be under pressure to explain away a financial black hole which will mean a reduction in student numbers of nearly half a million over the next decade. The electoral problems of seeing off a hollow promise to abolish fees are negligible. History suggests the electoral problems of being a broken-backed government, dependent on a party which has proved to be even more divided than Labour used to be in its worst periods - will not be.

This might be less persuasive if universities didn't matter as never before to the national economy or if this was not a genuinely social democratic policy which targets public funds where they are most needed: to poorer students and to provision for those currently blocked by poor child care and schooling from ever having the chance to get to university in the first place. The spectre raised by a defeat tonight is of a catastrophic funding gap not remotely plugged by the justly unpopular up front fees which will remain in place if the government does not win.

This isn't to make a case for prolonging a Blair premiership for the sake of it, any more than to deny the anger over the Iraq war which continues to inflame the spirit of rebellion in the Labour Party. It is to say that these are the worst possible circumstances and issues, for party as well as for the country, on which to engineer the replacement of the leader. Last night the vote still looked like the closest of calls; the nervousness in No 10 - slightly more marked than at Charles Clarke's Department for Education - partly reflects no doubt a bleak apprehension that Gordon Brown, now publicly in unequivocal support of the Prime Minister, wins either way. Either Blair is brought down in Brown's favour or he is rescued by, and therefore arguably in hock to, the Chancellor.

But there is no doubt that the latter is a better outcome for the party, for Blair, and for Brown himself. Whether Gordon Brown is going so far, as the experienced minister Lord Rooker suggested at the weekend, as to warn his several allies among the rebels that he will not have them in any future government he heads if they continue to defy the present one, can only be, to put it mildly, a matter for conjecture.

But whatever the short-term advantages to a Brown succession of a Blair defeat tonight, the legacy he would inherit would be a severely damaged one. What is at risk tonight is bigger than a premiership or even the New Labour project: it's the hard won reputation and future of Labour as a credible party of a government.

d.macintyre@independent.co.uk