Mr Howard's opportunism should not distract from the wider scandal of the Iraq war

The pro-war coalition is falling apart. Michael Howard now claims that had he known last March that the evidence of Iraq's weapons of mass destruction was unreliable he would not have voted for the Commons motion in favour of invasion. This smacks of political opportunism, but the Tory leader is justified in pointing out that everything has changed now we know that the Iraq intelligence presented by the Government was so flawed.

The pro-war coalition is falling apart. Michael Howard now claims that had he known last March that the evidence of Iraq's weapons of mass destruction was unreliable he would not have voted for the Commons motion in favour of invasion. This smacks of political opportunism, but the Tory leader is justified in pointing out that everything has changed now we know that the Iraq intelligence presented by the Government was so flawed.

The Prime Minister has an easy riposte for Mr Howard should the Opposition leader reiterate his change of mind in the Commons debate on the war tomorrow. Mr Howard's description of the deposition of Saddam Hussein as "necessary", "just" and "arguably overdue" in a speech only four months ago will be thrown back in his face, just as it was last Wednesday after Lord Butler's report was unveiled. The Tory leader has to explain why his party never voiced any scruples about the case against Saddam before and why it acted as the most enthusiastic cheerleader for Mr Blair's invasion, ensuring it won parliamentary support.

Of all the self-inflicted wounds inflicted on the Tory party by its former leader Iain Duncan Smith, the unquestioning backing for the invasion of Iraq has proven the most damaging. Iraq is a millstone around the Prime Minister's neck, shattering the sense of trust that the public once had in Mr Blair. No matter how he tries to shift attention back to public services, no matter how many five-year plans he unveils, he simply cannot draw a line under the issue. Last week's by-elections in Leicester South and Birmingham Hodge Hill demonstrated that there is little mood of forgiveness among the electorate.

But the Tories are powerless to exploit this glaring weak spot. Instead, Mr Howard finds himself desperately trying to attack the Prime Minister on Iraq from the pitifully small space his predecessor has left him. He claims he would not have supported the Commons motion that referred to Iraq's weapons of mass destruction but that he would have voted for a differently worded motion proposing to depose the Iraqi leader. This contorted position is unconvincing because, as Mr Howard well knows, the only legitimate casus belli under international law was the presence of WMD.

Although Mr Howard has ended his party's civil wars, some Conservatives may be looking wistfully in the direction of colleagues who opposed the war. Kenneth Clarke and Malcolm Rifkind (set to return to the Commons in the safe seat of Kensington and Chelsea) both questioned the rush to war and took issue with the Prime Minister's justifications for an attack.

But the Conservative leader may have some cause for hope. The US Senate Intelligence Committee uncovered failings by the CIA very similar to those identified by the Butler report in the case of MI6. The Senate committee's unambiguously tough report has enabled the Democrats, most of whom voted in favour of the Iraq war in Congress, to intensify their opposition to President Bush's Iraq adventure. If this approach helps the Democrats to win November's presidential election, Mr Howard would be foolish not to adopt it against Mr Blair. Yesterday's strong criticisms about the use of intelligence from the former UN weapons inspector, Hans Blix, and the former head of the Iraq Survey Group, David Kay, further ratchet up the pressure.

It is politically credible for the opposition on both sides of the Atlantic to claim that they were misled about the threat posed by Iraq, just as much as the general public, by two governments recklessly eager to invade. Mr Howard will be emboldened if Labour Party backbenchers also adopt this line in the Commons debate tomorrow.

The charge of opportunism that the Government has levelled against Mr Howard rings true over Iraq. But there is a bigger issue for the electorate to contemplate: the zealotry of the Prime Minister in the build-up to war, which increasingly looks like adding up to something substantially worse than the usual opportunism of politicians on the make.