Today, the House of Commons has a chance to prove its relevance to the nation

Above all it is the Labour MPs who have an opportunity, indeed a duty, to challenge their leader

For once a parliamentary event matters. Forget about what happens in the studios and in the newspapers. One way or another today's debate in the House of Commons on the Butler report will change the political weather.

In theory the stakes could not be higher. Before the conflict Tony Blair told MPs repeatedly that he had no doubt Saddam Hussein posed a current and growing threat. Lord Butler has concluded that the intelligence on which Mr Blair based his assertion was patchy and open to doubt. Presumably therefore MPs will want to discover the reasons for the discrepancy, the gap between what Mr Blair knew and what he chose to present.

This matters partly for the sake of the House of Commons as an institution. Quite often these days it is said casually that the Commons has ceased to matter. The observation is so common it has become a cliché. Both the cliché and our acceptance of it are alarming. After all, this is where we send our elected representatives. If the arena in which they perform is irrelevant where does that leave British democracy?

Mr Blair is not to blame for this. He does not ignore the Commons, an allegation often made against him. Indeed he is enough of a parliamentarian to have conceded a vote in advance of the war.

A longer Prime Minister's Question Time once a week is potentially as challenging as two shorter sessions, especially as a leader of the opposition has the opportunity to ask six questions rather than the three permitted under the old system. It is the MPs who are to blame for their failure or reluctance to hold him effectively to account.

In the case of the war against Iraq the House of Commons has been absurdly and dangerously unrepresentative. With both the main parties supporting the conflict Mr Blair is usually untroubled by his many visits to the chamber. The country is divided. Millions protest. Diplomats rage. Foreign Office officials privately despair. Some newspapers oppose the war. The House of Commons appears to be on another planet as it cheers Mr Blair's simplistic sub-Churchillian oratory with bipartisan gusto.

Today's debate is therefore important for Michael Howard as he seeks to liberate himself from the trap of unqualified support for the war. Mr Howard was highly effective last week on the day the Butler report was published. Now he has had more time to assess the paragraphs relating to the patchy intelligence and the way it was presented. Evidently Mr Blair is worried enough to ensure that the Conservative leader is rubbished more extensively than usual.

Peter Mandelson was one of several Blairites leading the charge of opportunism in the media yesterday. Of course Mr Howard is hugely and rightly constrained by his support for the war. Not for the first time Mr Blair led the Conservatives into a trap. He was for the war, so the Conservative leadership shouted their support even more loudly. A subtler Tory leadership would have offered much more qualified backing.

Even so it was very noticeable last Wednesday that Mr Blair did not answer a single point made in Mr Howard's short speech. Now Mr Howard has another opportunity to get some answers. This is as big an opportunity for Mr Howard as the debate on the Westland crisis in 1986 was for Neil Kinnock. As he has reflected ruefully since, Mr Kinnock blew it. Mr Howard will have no bigger opportunity this side of an election to unnerve his main political opponent.

But above all it is Labour MPs who have an opportunity and indeed a duty to challenge their leader. In reality the House of Commons was not removed from the rest of the world. Privately many Labour backbenchers and indeed ministers were alarmed by Mr Blair's timid determination to stand shoulder to shoulder with President Bush. Some of them put loyalty to the Prime Minister above their own convictions. Others voted for the war with an eye on their own future.

Some were swayed by Mr Blair's advocacy based on the intelligence. Inadvertently or otherwise they were led to believe that the intelligence was reliable in its evocation of Saddam's might. Are they going to sit there now cheering their leader as he declares with his usual intoxicating conviction that the war was the right thing to do?

The relatively muted response so far to the Butler report takes some explaining. I suspect it is based on two factors. In retrospect the BBC did Mr Blair a favour by reporting even more damning and inaccurate allegations, conjuring up cartoon-like images of Mr Blair and Alastair Campbell imposing intelligence on a reluctant Joint Intelligence Committee knowing it to be wrong. Politics is subtler than that and politicians are not so one dimensional in the way they behave.

Mr Blair's second piece of good fortune is the way many have viewed the Butler report through the Blair/Brown prism. The rivalry between the two of them is so intense that any politician or journalist who expresses concerns about the use of intelligence is dismissed as a Brownite. The idea that the Butler report may contain criticisms of substance is therefore easily dismissed. Instead the political story becomes more about whether Mr Blair plans to carry on in Downing Street rather than whether he should have the right to do so.

As far as his future is concerned, Mr Blair and his entourage are spinning energetically. Some government insiders believe that it was the Prime Minister himself who phoned the editor of The Sun 10 days ago to tell her that he planned to serve another five years. Evidently he has been informing other journalists that he is in a radical frame of mind.This is meaningless.

Mr Blair celebrates his radicalism even when he is being cautious and timid. He has done so for 10 years. Indeed it is one of his current ploys to regret his caution in the first term. Yet during that first term he was proclaiming his radicalism. Sketchy policies on welfare were described as a welfare revolution. The introduction of NHS Direct was hailed as a "revolution in healthcare". Mr Blair is always radical in the present and more cautious in the past.

He was up to these tricks again yesterday. Some sensible and fairly incremental proposals for tackling crime were hailed as the "end of the liberal consensus". Were Margaret Thatcher and Mr Howard part of that liberal consensus? More to the point were those old traditionalists, Harold Wilson and James Callaghan? If Mr Blair stays on for a few more years, he will no doubt look back at yesterday's launch as being unnecessarily cautious.

The same dynamic applies to the intelligence on WMD. In the past he used an argument to get his way. Now it is time to move on and forget about the arguments conveniently deployed.

It was the House of Commons that gave Mr Blair the go-ahead for war. Today MPs have a chance to discover what really happened. If they fail to do so voters will be right to conclude that Britain's elected chamber has ceased to matter.

s.richards@independent.co.uk