“Bush’s Brain” has finally left the building. Cynics might suggest the presidential grey matter has been missing for years, but the departure of Karl Rove from his privileged seat at George Bush’s side for the past ten years is a large milestone on the President’s long march into the political sunset.

Mr Rove has been among the most revered and despised of political figures since he emerged as the Republican presidential candidate’s chief strategist in the 2000 election. He was the “architect”, the moniker Mr Bush himself bestowed, of the narrow election victory that year and the less contentious one four years later. Perhaps even more important, he was the designer of the controversial President’s entire governing strategy. Mr Rove read and understood brilliantly the polarised political mood of America in the Bill Clinton years of the 1990s.

He devised a plan that eschewed the traditional approach for political success in a democracy – governing from the Centre to attract as broad a range of support as possible – and instead fashioned what has been called “the 51 per cent solution”.

It was a consciously partisan style that aimed to energise the conservative political base of the Republican Party to ensure that it turned out to vote in sufficient numbers to edge out the Democrats.

After September 11, 2001, Mr Rove was crucial in advising the President not to reposition himself as a unifying figure but to press home the large Republican advantage on national security.

For Mr Rove, a keen student of American political history who can quote returns from 19th-century midterm elections in the way some people can recall baseball statistics, the ultimate ambition was a large one. He saw Mr Bush as emulating William McKinley, the President exactly a century earlier who ushered in an era of Republican dominance in American politics.

To be fair to Mr Rove it was not political judgment but administrative incompetence that led to the collapse of those hopes. The strategy might have worked had it not been for the Administration’s disastrous failure in Iraq, a policy for which he can hardly be blamed.

And yet the problem with a narrow, 51 per cent approach to governing is that there is little margin for error. When things go wrong they go spectacularly wrong.

The Iraq catastrophe turned a large majority of Americans against the Republicans, and propelled the party to its biggest defeat in 14 years in the 2006 midterm congressional elections. Far from ushering in an era of Republican super-domination, Mr Rove faces the bleak probability that the Democrats are facing their best political conditions in a generation.

And the once solid conservative base that Mr Rove stroked and fed has turned angrily against the President. The past two years have marked not just a public disenchantment with the President and his chief adviser, but a conservative counter-revolution against them.

Mr Rove’s departure is likely to be less the cause of a new direction in US politics and more a symptom of a shift already firmly in place.

His last hurrah was probably the attempt this year to get an immigration reform Bill passed through Congress. Mr Rove, ever the cunning political strategist, has long seen the Hispanic vote as key to building a long-term advantage for the Republican Party. Latino voters – mostly Catholic in belief and traditionalist in values – are natural conservatives, in his view.

Mr Bush has long worked, with considerable success, to bring them into the Republican fold. By supporting an immigration reform that would have granted a form of amnesty to 12 million illegal immigrants, mostly Hispanic, Mr Bush and Mr Rove hoped to build a real long-term base.

But the proposal met with anger from conservatives, who feared out-of-control immigration with large economic and cultural consequences, and the Bill went down to defeat in Congress, with only a handful of Republican backers. With little more than a year left in his second term, Mr Bush’s chances of getting any other domestic initiatives through Congress are virtually zero. Mr Rove, like so many other senior Administration officials, is doubtless exhausted by life in the White House and has simply decided there is not much more he can do.

Where does his exit leave Mr Bush? More focused than ever on his foreign policy is the answer. For the last year or more, there has been a sharp tension between Mr Bush’s political objectives and those of his party. Most Republicans would love to find a way to avert catastrophe in next year’s presidential and congressional elections and urgently want a way out of Iraq.

Mr Bush remains focused more than ever on his legacy and is determined to leave office with some evidence of stability and progress there, even if it means prolonging the American engagement.

In the end, not even a man with a political brain supposedly large enough for two could find a way to square that political circle.

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