One vote at a time. All the attention at present is on next week's Commons vote on extending the maximum period of pre-charge detention from 28 days to 42 days. Will Jacqui Smith's concessions be enough to win over sufficient numbers of Labour MPs? But even if, or rather especially if, the Government wins the vote, the story won't end there. Far from it.

The politics are complicated. Despite Ms Smith's reportedly strong performance at the meeting of the Parliamentary Labour Party on Monday, the outcome of the vote is far from certain. The government whips are busy - “Now is the time to come to the aid of the party,” as one veteran “persuader” remarked to me. Although a few MPs have said publicly that they will back the amended plan, many have not yet committed themselves. Don't forget that Gordon Brown has spent more time this week on Northern Ireland than any other issue, and the fate of the Democratic Unionists' nine votes has still to be resolved.

The Government's problems in winning the argument are underlined by a highly critical report by the Joint Committee on Human Rights, a group of MPs and peers who tend to be strongly libertarian. Andrew Dismore, the committee's Labour chairman, said that the additional safeguards were “inadequate to protect individuals against the threat of arbitrary detention”. Ministers emphasise the complexity of terrorist plots, and the time needed to go through computer disks and records, as well as the nightmare scenario of simultaneous plots and attacks.

After lots of rumours, hints and reassurances about the exceptional nature of the provisions, appeals to loyalty etc over the next few days, the outcome is likely to be close. If the Government loses, the issue will disappear, but only for the time being. Proposals for an extension beyond 28 days are likely to be revived at some stage (remember, MPs defeated the 90-day limit in November 2005).

If the amendment goes through, it is likely to be by less than 20 votes. So the Lords will not feel constrained to accept the decision. The second chamber has repeatedly challenged the Commons/ Government on terrorism and detention. Opposition to the plan comes not only from Conservative and Liberal Democrat peers. Criticism has also come from Lord Goldsmith, the former Attorney-General, and Lord Falconer of Thoroton, the former Lord Chancellor. In addition, I know of several normally loyalist Labour peers, including former ministers, who will be absent on the day. So it may be a very large defeat, not a narrow squeak. The scale of the Labour revolt will make it harder for the Government to play the “Elected Commons v Unelected Lords” card and to reverse the change. Ministers could invoke the Parliament Act to push the Bill through in a year's time, but that would be messy.

In one sense, the politics are very crude. A further terrorist outrage, or even a foiled plot, could tilt the balance of opinion, in the Commons at least, regardless of the specific merits of the issue. No leading politician will want to be seen to be “weak” on terrorism.

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