Something quite strange happened in Washington today. Three US Senators took a day off from their usual working routine and showed up in the US Senate.

John McCain, Hillary Clinton and Barack Obama swapped for a day the life of campaign flights across the country, adoring crowds in airport hangars and soft-focus interviews with television chat show hosts for a brief trip back to their regular place of work.

The man who forced the remaining US presidential candidates to make this sacrifice was General David Petraeus, the commander of US forces in Iraq, who was giving his long-awaited progress report on the war to two legislative committees.

General Petraeus, along with Ryan Crocker, the US Ambassador to Iraq, spoke to the Senate Armed Services and Foreign Relations committees. While television coverage dutifully recorded the exchanges between the general and all the lawmakers on the dais, the main focus was on the interaction with the presidential hopefuls.

For the candidates it was a rare opportunity to listen to and ask questions of the man whose military operations provide the backdrop to the presidential election campaign.

For General Petraeus it was a rare opportunity to make his case to the next president of the United States – even though he could not be sure which one of the three senators would be facing him in the Oval Office from next January.

The curious paradox behind yesterday’s made-for-television show is that the general is probably pivotal to the outcome of the presidential election in November; yet nothing that is said in the heat of the campaign is likely to be very relevant to what actually happens after President Bush leaves office early next year.

Mr McCain, the presumptive Republican nominee, has made his support for the general’s leadership of the surge in US troops in Iraq over the last year the cornerstone of his presidential hopes. He insists that, whatever errors had been made previously, the US is now finally succeeding in Iraq and that any premature retreat from Iraq will leave the country and US foreign policy in ruins.

Both Democrats – who confine their serious political differences to such issues as the frailty of Mrs Clinton’s memory and Mr Obama’s relationship with fiery preachers – essentially do not differ on Iraq. They both argue that the surge was another error, that the US is not truly achieving greater security and that American forces should be rapidly withdrawn.

The general was careful yesterday not to be seen to be over-selling the surge. There was not a hint of triumphalism or claims of Mission Accomplished. Instead he said the gains so far were “fragile and reversible”. Though he outlined political advances as well as security improvements in Iraq in the last six months he was especially wary of overstating progress in the light of the somewhat unexpected events in Basra in the last month.

But the essence of his message was that the US should not pull the plug any time soon. Not only was Iraq still too vulnerable to sectarian and intra-sectarian violence, but Iran’s growing role in the country was also a direct threat to the US.

Mr McCain echoed the general’s caution, expressing his support for continuing the US role, while Mrs Clinton repeated Democratic scepticism about the value of keeping US forces in Iraq indefinitely.

But the hallmark of this entire debate is its continuing unreality.

Republicans seem to suggest that the war is going so well that the US should simply stay indefinitely. But senior strategists close to Mr McCain acknowledge what many in the Pentagon are saying with increasing alarm - that the strains the war is placing on US military capabilities are so great that some significant reduction in the American role is essential some time soon.

At the same time, Democratic foreign policy advisers also admit that the chances that their candidate will be able to meet campaign promises and pull US forces out quickly next year – whatever the situation on the ground in Iraq – is equally absurd.

The reality is that, once the posturing is done and the election is over, whoever wins is going to have to sit down with General Petraeus or his successor – without the television cameras - and figure out a pragmatic resolution to this messy and prolonged American engagement.

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