The departure from the White House of Karl Rove signals definitively that in terms of domestic ambitions - the desire to win a final, major legislative victory - the Bush presidency is over.

In Rove, Mr Bush is losing not only the most brilliant and influential political adviser of modern times but also his closest and longest serving friend inside the Administration.

Mr Bush's domestic agenda effectively ended with the defeat of his immigration reform Bill earlier this year. It is highly unlikely that Mr Rove would be leaving now if there was a chance of any more significant legislative achievements before Mr Bush leaves office in January 2009.

Mr Bush has 17 months left in office, a still crucial period for his foreign policy - particularly in Iraq, Iran and the Middle East. It is an area that, despite a Democrat-controlled Congress, he is far from a lame duck.

With Mr Rove back in Texas, the President will now rely more heavily than ever on Condoleezza Rice, his Secretary of State and the last remaining major figure of his Old Guard, and Robert Gates, his understated but widely respected Defence Secretary, who also served in his father's administration.

With Mr Rove's departure, Democrats will also be quietly mourning. They have been obsessed with Mr Bush's political strategist for years, and are losing a figure inside the White House with whom they invested a sinister, Svengali-like power – and thus a man who excited grassroots activists with a fear and loathing that at times even eclipsed that reserved for Dick Cheney.

Every dirty trick, every mishap for any political opponent, was always laid at Rove's door. "I'm a myth. There's the Mark of Rove," he said today.

But Rove was brilliant, ruthless, and without doubt master of the political smear - and with the cunning to cover his tracks. Democrats are right about one thing: he was at the centre of virtually every policy decision, from the choosing of Supreme Court justices and foreign policy decisions to the appointment of obscure diplomats.

Before the invasion of Iraq in 2003, he played a key behind-the-scenes role in persuading Congress to back the war, and chaired meetings with congressmen and senators inside the White House.

Rove ruthlessly and brilliantly fused politics and policy. He masterminded the strategy of connecting in the public mind the September 11 terror attacks and the invasion of Iraq. It was the central plank of Mr Bush's 2004 re-election victory, the first time since 1936 when a president won re-election while helping his party gain seats in both houses of Congress.

Until the 2006 loss of Capitol Hill to the Democrats, Rove exploited the spectre of terrorism with enormous political success. As late as last week he outfoxed the Democrats – after convincing moderates that they would look soft on terrorism – and successfully won Mr Bush expanded warrant less wiretapping powers in Congress.

But Rove's greatest genius was in winning elections. He had an encyclopaedic knowledge of the demographics of every county – and in battleground states even every district – and established the Republicans' massive and effective voter target operation, using the data mining skills he gleaned as the head of a direct mail company. He spotted - and nurtured - the growing power of the Religious Right, the cornerstone of Mr Bush's political success.

He leaves having survived an exhaustive criminal investigation into the role he played in the Valerie Plame CIA leak scandal – he was never indicted – a barrage of congressional subpoenas over the firing of federal prosecutors, and happy in the knowledge that all the Democrats' efforts to bring him down never succeeded.

His stated reason for going now - to spend more time with his family - should be taken at face value. He has been with Mr Bush for 14 years, in the White House for 79 months, and at the heart of a tumultuous presidency that began with the 2000 Florida recount and sits today mired in Iraq.

He took a young, brash George W Bush and turned him into one of the most successful politicians in modern times, charting in minute detail his journey to the Texas governor's mansion and then two terms in the White House.

Although his claim after the 2004 victory that he had played a part in creating a permanent Republican majority now looks overblown, the American political landscape has fundamentally changed in the past decade and Mr Rove is one of the major reasons why.

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