There are always two separate stories to be told on the day of an important parliamentary vote. The first is about what will change in the country, and the second about the endless drama of party fortunes.

Tonight’s vote brings the two stories together in a way few others do. There’s a certain irony to the fact that some of the thirty or so Labour MPs prepared to vote against the government won’t do so simply because Labour is so unpopular. Were it standing higher in the polls, and Mr Brown’s leadership not being called into question every twenty minutes, they would almost certainly vote against the proposals.

But Labour is not doing well in the polls, and Gordon Brown’s authority is a tenuous, easily fractured thing. And so it is that Labour MPs will spend today weighing up their duty to their party against their duty to their conscience.

The story of party fortune is a relatively simple one. If Brown wins tonight the attacks on his leadership qualities will die down a little bit, for a short period of time. Barring unforeseen events or an exceptional run of luck they will be back, probably around the time of the party conference in September. If he loses, it will be one more nail in a coffin that is increasingly covered in them. Regardless of Downing Street statements disassociating tonight’s vote from Mr Brown’s position, the chances of a leadership bid will multiply.

The story of this country’s fortunes – the story historians are interested in – is neither simple nor short. In response to the constant danger of terrorism, Britain will have adopted one of the most draconian anti-terrorist systems in the western world. The events of September 11th changed the politics of every country, but there are good reasons to argue that the changes it has provoked in Britain are more extensive and deep-seated than anywhere else.

There are many ways it could turn out. It is quite possible the powers will never be used, although that seems unlikely. Alternately, they may well be used but only in a restrained and reasonable manner. Jacqui Smith says they will only be activated by a “grave and exceptional threat”, and it’s quite possible that phrase will be interpreted in a narrow and specific way – during a real and sustained attack to Britain or her interests overseas.

But the signs don’t point that way. Firstly, if the phrase was to be interpreted that way by the executive, why not construct it in a narrower sense, such as the human rights committee’s “public emergency threatening the life of the nation”? Historically, the executive has a tendency to submit vaguely worded legislation in order to give itself room to manoeuvre in the future.

Also, the government’s record on defining terrorism is deeply unsettling. One only had to witness the arrest of elderly pacifist Walter Wolfgang under terrorist legislation at a Labour conference two years ago to know that these laws are not always being directed at people any normal person would classify as a terrorist.

There is a strong chance these powers could be used to imprisoned small swathes of the Muslim community – up to and including teenagers experimenting with unpleasant and extremist books. Suddenly, Britain would not look like the country we live in today.

Tonight’s vote will help define the fortune of the Labour party and specifically Gordon Brown. But it is also one of those rare moments in parliament when Britain decides what kind of country it will be over the coming years.