This idea came to me the other day as to what would have been the implications if the Tories had failed to support Blair over Iraq. Some Tory MP's did vote against the Commons motion supporting the War and IIRC 2 of their junior frontbenchers resigned so they could vote against it.
Obviously this would need a different Leader from IDS, perhaps Ken Clarke or even David Davis? How plausible is it that the Tory Leader at the time tells Blair during that debate "I'm sorry Prime Minister but I am not convinced by the case for war that you have set out and therefore I cannot support it. I have decided to allow all my MP's a free vote on this matter so they can follow their consciences." IIRC the Commons vote was won pretty clearly despite a massive Labour rebellion, had more Tories gone through the No Lobby then the numbers could have got even tighter for Blair possibly damaging his authority.
It could also have made the run up to the 2005 GE much harder for Blair, a Tory Leader who had opposed the War would have been able to attack him much harder about the failure to find any WMD, the "dodgy dossiers" and the catastrophic failure to prepare for the aftermath. This could have helped deny Charles Kennedy from claiming that ground for the Liberal Democrats.
So how would this have affected the election? Could the Tories have performed better than IOTL and at least cut Labour's majority down even more? What other consequences could have happened?