By Nick Assinder
Political Correspondent, BBC News website
Tony Blair is making no bones about the importance of his education bill, telling his MPs the reforms represent a "defining moment" for his third term government.
But it is also no exaggeration to say that the prime minister's future will also be sealed during the bill's passage through parliament.
Blair faces biggest education test
Not because he is likely to be defeated when the proposals eventually go before MPs.
Even backbench rebels expect the prime minister to win the day in the crucial Commons votes.
Although it would still be rash to claim victory this early in proceedings, the concessions included in the bill will have satisfied a number of the potential rebels.
But this time the danger will come from the way that likely victory is secured and at what cost.
Specifically, will it only be achieved with the support of the Conservatives and in the face of widespread opposition from his own parliamentary party, as some believe is likely?
That would inevitably lead to claims Mr Blair has lost his authority over his own side and is operating a Conservative-style agenda. There would even be calls for him to stand down.
The prime minister is well aware of that danger and has told his MPs he wants it to be a "Labour" bill. But, to the dismay of many of his rebels, only last week he indicated he was ready to accept victory as a result of Tory votes.
And the published bill also shows no new concessions to further buy-off backbench rebels.
But victory thanks to the Tories would still leave deep wounds on his own benches and the longer-term impact on his ability, or even willingness, to stay in Downing Street could be severe.
Alternatively, might victory come after the prime minister has made so many concessions to his rebels that his radical vision enters the world with a whimper rather than a legacy-securing big bang?
That would certainly see claims he had retreated in the face of his rebels. It might even speed up his departure. But a final verdict on that will only come after MPs have considered the fine print of the weighty bill.
Blair and Kelly have offered concessions
Worst of all - and least likely - will the education reform package fall horribly between two stools and be so watered down as to be feeble and, as a result, lose Tory support and fail?
That too would undoubtedly lead to demands for him to go, which would very likely prove irresistible.
Needless to say, the prime minister is confident he will win the day with a radical package of measures that will transform the education system and raise standards across the board.
He believes he has offered only sensible concessions which have left the heart of his proposals intact and, on first reading of the bill, many have claimed that is indeed the case.
And the prime minister seems confident that he might just win with a majority of his own MPs on-side.
Such an outcome would not only secure his reforms but underpin his premiership and leave the power to choose the method and timing of his departure firmly in his own hands.
Now the bill has been published, all sides will trawl through and it will only be after the contents have been thoroughly digested that we will get a clear idea of where this most important of reforms will leave Tony Blair.