The Week in Politics: Momentous week for Blair - but it could be his last great escape

Whenever Tony Blair is in trouble, there is a standing joke in Downing Street as his aides survey the morning newspapers: they poke fun at the reports saying the Prime Minister faces the "worst crisis" or the "worst week" of his premiership.

But the aides know that, for once, the media is not overegging the pudding as it looks ahead to what will be the most momentous week since Mr Blair won power in 1997. At about 12.30pm on Tuesday, the two issues which, between them, threaten to terminate his Downing Street tenancy will simultaneously reach a climax. Just as the House of Commons starts to debate plans to allow universities to charge top-up fees of up to £3,000 a year, No 10 will take delivery of its advance copy of Lord Hutton's report into the death of the Government scientist David Kelly, to be published the following day.

For the Blair aides preparing for the reckoning on both fronts, there is only one period that compares to it: the build up to the critical Commons vote on the Iraq war in February, when the Prime Minister was much closer to the edge than we realised at the time. Some advisers believe - and hope - that those frenetic few days will remain the nadir of his premiership. After all, there are few more serious decisions than sending British forces to war. But they know it could be even worse next week.

In February, Mr Blair's fate did not lie in his own hands, but in those of Labour MPs. In the next few days, it lies with his backbenchers and a 72-year-old recently retired law lord. Of the two colossal challenges, my feeling is that the vote on top-up fees has the potential to do more damage. Defeat on Tuesday would not mean the end of Mr Blair's premiership. But it would be the beginning of the end. The Government would easily win the formal vote of confidence that would follow, probably on Thursday; even those Labour MPs who no longer want Mr Blair do not want a general election. But defeat on his central agenda of reforming public services would leave Mr Blair one of the walking wounded. When that happens, political death is only a matter of time, even if it is a lingering one.

The mood in Downing Street yesterday was pretty grim. Mr Blair is hitting the phones this weekend, trying to cajole 25 of the 100 rebels into supporting him - the difference between defeat and victory. It is not a piece of Labour spin to say the Government is in real trouble over Tuesday's vote.

Mr Blair's closest aides know they have been here before - over Iraq and foundation hospitals. They feel they can win on Tuesday but can't yet see how the numbers will add up. Despite the headlines about MPs switching sides to back the Government, there has only been a trickle of one or two a day and time is running out. As one Blair ally told me yesterday: "Just because we survived before, it doesn't mean we'll win this time. At some point, it won't work and we will lose."

The language was instructive. What might not "work" is Mr Blair's tactic of turning every important Commons vote into a vote of confidence in his leadership. Several MPs will vote against Mr Blair for this reason on Tuesday, rather than against top-up fees. Another group of rebels will not budge because they believe Labour must not break its 2001 election manifesto promise to block top-up fees. The biggest group of rebels are genuinely opposed to variable fees. Mr Blair's best hope is that enough of them will be persuaded to back off by last-minute concessions before Tuesday, knowing they could try to amend the Higher Education Bill during its later stages. Mr Blair would be happy enough to survive to fight another day.

One argument that ministers are using with the rebels is that defeat would send a signal that Labour had reverted to its bad old ways. "If Howard wins this vote, he could win the general election," the dissenters are being told. So the vote on top-up fees is more serious, and probably more salient with the public, than the Hutton report. In the past week, Downing Street's energy has been expended largely on top-up fees, because it cannot really do much about Hutton until the advance copy lands on Tuesday. That does not mean there are not real concerns about Lord Hutton's verdict. If Mr Blair is blamed for the "naming strategy" under which Dr Kelly's name became public, there will be calls for his resignation - and he will be much more vulnerable if he has lost Tuesday's vote. If not, the media's hunt for a scalp will probably turn to Geoff Hoon. Ministers hope the Hutton report will spread the blame between the Government, the BBC and Dr Kelly. But I suspect the media will not easily be diverted from Lord Hutton's judgment on the Government.

Will Mr Blair come through his 24 hours of hell? My hunch - and it is only that - is that the Great Houdini will somehow wriggle free once again, squeaking victory on top-up fees and surviving whatever arrows Lord Hutton fires at him and his government. But it is going to be too close for comfort, and it might be his last great escape.

a.grice@independent.co.uk