At this late stage in an American presidency, even in the most favourable circumstances, even for the most popular incumbents, lame duck is definitely on the menu.

These are hardly the best of circumstances and this is hardly one of the most popular incumbents. With little more than a year to go to the end of George Bush’s presidency, his approval ratings stand near historic lows at just above 30 per cent. Last November his party lost control of both houses of Congress.

The death march of senior officials out of the Administration, routine around this stage of a second presidential term, has become a stampede. Karl Rove, the top White House aide, the Cardinal Richelieu of the Bush presidency, has gone. Alberto Gonzales, the Attorney-General, the Harpo Marx of the Bush presidency, will be gone in a few weeks.

By now Mr Bush should be a governing irrelevance, a liability to his party, the object of scorn and derision. Every Republican candidate with an ounce of instinct for self-preservation in his blood should be running away from the President as though he were a burning building.

But what is this? Next week Mr Bush seems certain to score one of the most important political victories of his presidency. General David Petraeus, the commander of US forces in Iraq, will testify before Congress, along with Ryan Crocker, the US Ambassador to Iraq, on the progress of the “surge” Mr Bush ordered earlier this year to much domestic political opposition.

A couple of months ago this event was viewed as a kind of D-Day in reverse for the war in Iraq. Democrats are solidly against continuing the war effort now — the party is united in demanding at least the start of a withdrawal. The ignominious British retreat from Basra has only solidified that position. A growing number of Republicans have broken ranks to indicate they, too, want an end to the war.

Although General Petraeus was always likely to give a guardedly optimistic report about the surge, the politics seemed increasingly hopeless for the Bush team. The Democratic majority in the Senate was backed by the momentum of overwhelming public opinion. And yet Mr Bush now looks just about certain to get his own way on Iraq. Not enough Republicans are ready to jump ship. Although the Democrats have a majority, they would need about a third of the Republicans in the Senate to vote with them to overcome a presidential veto. They don’t have those votes. So the surge will continue.

This unexpected turn of events reflects in part, of course, the good news out of Iraq. Even the war’s fiercest critics have had to acknowledge that real progress has been made in the last few months. In Anbar province and elsewhere Sunni insurgents have united with the Americans against al-Qaeda terrorists. The increased security effort by US forces has created the space to build the foundations of a political settlement.

But the surprising endurance of Mr Bush’s political leverage reflects something much more than that — the reluctance of Republicans to break with their President and the revolutionary foreign policy he unleashed five years ago.

Perhaps even more striking than next week’s probable Iraq victory in Congress is the way in which the leading Republican candidates to replace him have cleaved carefully to the Bush policies of the last few years. Some Republican strategists have urged the candidates to “do a Sarkozy” and win by running against the record of the unpopular incumbent president of his own party. So far, none of them has taken the bait.

To some extent it underscores the fact that, despite everything in the past four years, despite Iraq, Hurricane Katrina and countless debacles, Mr Bush remains highly popular among the Republican base. Polls indicate he enjoys an approval rating of about 80 per cent among Republican voters. The war, especially, remains well supported by Republican voters.

The electoral mechanics of America’s primary system — in which candidates first battle to get selected by each party’s own voters – have created a dynamic that keeps the Republican contenders on the straight and narrow.

But the danger is that the straight has become very narrow. These days only about 35 per cent of voters describe themselves as Republicans – down from more than 45 per cent ten years ago. You don’t have to be a maths genius to recognise that 80 per cent of 35 per cent is not much of a basis for a majority in the country. Many more Americans describe themselves as Democrats, and independent voters lean heavily away from the Republicans.

A fascinating poll by the Republican pollster Tony Fabrizio in The Wall Street Journal this week showed that, as they have shrunk in number, Republicans have become more conservative, creating the powerful impression that the Republicans are a rump, a marginalised bunch made up largely of grumpy old men.

And yet, there may still be salvation in the works. First there is national security. Voters beyond the Republican base still lean towards the Republicans on foreign policy and defence. They blame Mr Bush for his incompetence over the war, but they do not necessarily reject a muscular foreign policy. They certainly don’t like the sound of surrender that seems to emanate from some Democrats these days.

The other piece of good news for the Republicans is that, even as they are currently staying close to President Bush, the leading candidates to succeed him are all of a quite different stripe. Not one — Mitt Romney, Rudolph Giuliani, John McCain or Fred Thompson — fits the mould of a rock-ribbed traditional Christian conservative.

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