The Prime Minister's strategy on Afghanistan is under mounting pressure from all three of the main parties as the political consensus on the war begins to crack.
The Conservatives and Liberal Democrats are preparing to call for the Afghan election to be re-run amid growing evidence of vote-rigging and intimidation. Their move puts Gordon Brown in a difficult position, because Hamid Karzai, the Afghan President, is utterly opposed to holding another election.
And Mr Brown is trying to prevent Labour activists staging a debate at this month's conference demanding that British troops be withdrawn from Afghanistan. Tensions over the war could dent Mr Brown's plans for a show of Labour unity at the last annual gathering before the general election.
If Mr Cameron becomes prime minister, he would send more troops to Afghanistan so that the training of the Afghan army and police could be quickened – and British forces withdrawn more quickly. His approach is described as "send them in, train them up and get out" in Tory circles. Senior Tories believe such a strategy is backed by Britain's military chiefs, whose call for up to 2,000 extra troops was blocked by the Government earlier this year. An extra 900 were sent for the Afghan election period only.
Although the Opposition still supports the mission in Afghanistan, Mr Cameron broke ranks with the Government yesterday by condemning the way the country's elections were held.
In a conversation with the shadow Foreign Secretary William Hague picked up by a BBC microphone, Mr Cameron said: "The things that seem to have happened are so naked, you know, you just saw the number of votes and the number of people who actually turned up at polling stations, it just couldn't possibly be right ... We should be very clear about that."
The Tories are expected to call for the election to be re-run after the Independent Electoral Commission has published its findings on the first round of the contest.
The Liberal Democrats called for a second round whatever the official result of round one because there were so many doubts about the process. Nick Clegg, the party leader, said: "It now seems very clear that the elections in Afghanistan have been plagued by fraud and we need a second round to establish some credibility in any government. This is necessary to ensure Afghanistan gets a president with legitimacy without which the conflict against the Taliban will be all the more difficult."
Ministers worry that Mr Brown's autumn fightback is being hampered by controversy over Afghanistan and the release of the Lockerbie bomber.
Yesterday the Prime Minister gave ground to his critics by calling on world leaders to discuss an exit strategy in Afghanistan. He called for "new benchmarks and timelines" to be agreed at an international conference later this year, in a joint letter to the United Nations with the French President Nicolas Sarkozy and the German Chancellor Angela Merkel.
Senior Labour sources confirmed that the party's high command is trying to head off a special debate on Afghanistan at the Brighton conference. But the grassroots Campaign for Labour Party Democracy has circulated a motion to constituency parties urging them to call for a "contemporary issues" debate, stating: "The Government should withdraw British troops from Afghanistan."
In a message to local parties, the campaign says: "Rather than keeping terror from the streets of Britain, the war is fuelling hatred and increasing the possibility of future attacks. Afghanistan must be guaranteed a future without the threat of war and foreign domination. Our Government should bring the British troops home immediately."
Labour officials are pressing the trade unions, who, like party members, hold half the votes at the conference, not to call for a debate on Afghanistan. A party source said: "There is concern at the top [of the party] about this. But there is also growing concern among party members about what is happening in Afghanistan."
The Prime Minister's spokesman admitted people were "anxious" to know the result of the elections but said it was important to let the counting process run its course, with only preliminary results announced so far. Describing reports of fraud as "no surprise", he added: "We always knew there would be potential difficulties with these elections and that fraud was a possibility."
Mr Hague said: "If the commission requires some elections to be re-run, that should happen. Nor should a full second round of the election be ruled out if that proves necessary."
Mr Brown, Mr Sarkozy and Ms Merkel said their proposed UN summit should set "new prospects and goals" for governance, rule of law and human rights in Afghanistan, as well as security and social and economic development." They added: "We should agree on new benchmarks and timelines in order to formulate a joint framework for our transition phase in Afghanistan, ie to set our expectations of ownership and the clear view to hand over responsibility step-by-step to the Afghans, wherever possible."
Fighting talk: Why politicians support war
It is a brave politician who opposes a decision to go to war, no matter how disastrous it proves to be in the long run.
The first war to be covered effectively by the British press was the shambolic campaign in the Crimea in 1853-56. Two MPs, Richard Cobden and John Bright, vociferously opposed the whole adventure and they both lost their seats in 1857. Not even the scandal of the Charge of the Light Brigade saved them.
In 1914 James Ramsay MacDonald led the call for a negotiated peace with Germany. He lost his position as chairman of the Labour Party, was sent white feathers through the post and at the end of the Great War, when he tried to stand for re-election in Leicester, was heavily defeated.
In 1956, the Labour Party, led by Hugh Gaitskell, opposed the decision to reclaim the Suez Canal by force. Suez was a disaster, the Prime Minister, Anthony Eden, had to resign, but the Tories won the next election with a thumping majority.
During the Falklands campaign in 1982, the Labour leadership constantly urged Margaret Thatcher to negotiate. Public opinion preferred a military victory, despite the cost in human lives. The Falklands War was a major contributor to Labour's humiliation in the subsequent election.
The Conservatives supported each of the wars into which Tony Blair sent British troops, including Afghanistan and Iraq. But they changed tack within a year of the invasion of Iraq, after the Liberal Democrats had shown that being anti-war can be a vote winner. In February 2004, the Tory leader Michael Howard called on Mr Blair to resign for allegedly sending the country to war on false information, and in July he implied that he would have voted against the war if he had not been misled by the Prime Minister.