Tony Blair is approaching one of the biggest moments since he became Prime Minister. At 7pm next Wednesday, he will enter the same division lobby as David Cameron when the Commons votes on the Education Bill, while about 50 Labour MPs oppose the measure.
Thanks to Conservative support, the Bill will get a second reading. Mr Blair will issue a "business as usual" message at a press conference the following day, saying his school reforms have won the backing of the overwhelming majority of his MPs.
However, something in the Westminster air will have changed, and a Rubicon crossed. The vote, if coupled with poor local election results in May, could shorten Mr Blair's Downing Street tenure.
True, the number of Labour rebels will probably be smaller than over the Iraq war and university top-up fees - but, unlike the school reforms, neither was a manifesto pledge. I can't recall a government passing a flagship Bill with the support of the main opposition party since 69 pro-European Labour MPs defied a three-line whip in 1971 to enable Edward Heath, the Conservative Prime Minister, to take Britain into the European Economic Community.
Mr Blair has already changed the terms of trade of British politics and may do so again on Wednesday. He won't lose sleep over the cries of "Ramsay MacBlair" when left-wing MPs compare him to the Labour Prime Minister who hopped into bed with the Tories and Liberals in 1931.
But there is a sense of foreboding in Labour's ranks. Normally loyal ministers feel uncomfortable about the Tories' warm embrace. Some fear Mr Cameron will pull the rug from under Mr Blair during the later stages of the Bill's passage on the grounds that ministers have watered down their proposals to appease Labour's rebels.
If the Tories made common cause with the Labour dissidents, the measure could be defeated on its third reading and that could be the end for Mr Blair. "It would mean regime change," said one cabinet minister. Another minister said Mr Blair's self-styled "high-wire act" had left him "at the mercy of a yank on the wire by Cameron".
Mr Cameron, who completes 100 days as Tory leader on the day of the vote, may regard opposing the Bill as a flip-flop too far. But Labour whips, ultra-cautious these days, are taking nothing for granted.
For Mr Blair, what matters most is not the parliamentary arithmetic but that the Education Bill is passed to show that Labour has not given up on reform. But he is convinced that the Bill's failure would give booster rockets to the Tories' strategy for the next election - to complete the reforms Mr Blair knew were necessary but Labour flunked.
So the stakes are high. A source close to the Prime Minister told me: "The question is who inherits the New Labour mantle - Labour or the Tories?"
On both sides of the Labour debate, the detail of the school reforms has gradually become less important. It is the big picture that matters. Some potential rebels have been won round by concessions and will support the Government, although some may oppose the Bill at later stages of its passage unless there are further changes.
Other critics are unmoved by safeguards over the selection of pupils and the role of local authorities, even though many Labour town hall leaders have now rallied behind the measure. For some MPs, the Bill has become the vehicle for them to register a general sense of disenchantment with the Government - or to get Mr Blair out.
When Ruth Kelly, the Education Secretary, held one-to-one meetings with about 20 backbenchers in recent days, the most common grumble about the Bill was: "I don't like the fact that the Tories are supporting it."
Full marks, then, to Mr Cameron, who has played a blinder on the issue so far. By saying that Mr Blair's independent trust schools would be a return to the Tories' grant-maintained schools - which they wouldn't - he fuelled the Labour rebellion.
But the Tory leader needs to keep his troops in line too. Some Tory MPs detest his strategy of hugging Labour close and would prefer a full-frontal attack to defeat the Bill. They don't like Mr Cameron's opposition to more grammar schools and fret about backing a Bill that will outlaw selection and parental interviews.
When Ms Kelly opens Wednesday's debate, she will insist that it is a "Labour Bill". But some ministers admit the measures have been sold badly. "At first, we implied they were radical right and would increase selection. Now we are doing a soft sell and saying the opposite," said one.
Some Labour rebels believe Mr Blair, on the last lap of his premiership, is bloody-minded and beyond caring. That would be to misread his mood. He is convinced that, if Labour gets on the wrong side of the argument over public service reforms, it will lose power. He knows that his much-vaunted legacy will be judged by others and he can't write it himself. The legacy he wants most of all is a fourth consecutive Labour victory.
Far from being semi-detached, as some critics claim, Mr Blair is still very focused. But he acknowledges a huge paradox.
He came into office in 1997 with strong political instincts but having never been junior minister for paper clips, not knowing how to play the system and get things done.
Almost nine years on, he feels that he has never been more sure about what works, what more needs to be done and finally knows how to achieve it.
But he is running out of time to do it.Reuse content