Karl Christian Rove, the most influential presidential strategist in modern American history, was born in Denver, Colorado, on Christmas Day, 1950.

His family life was traumatic. On his 19th birthday, his father, a geologist, walked out of the family home in Sparks, Nevada, and never came back. Then the nerdy, wonkish student found out that his father wasn't even his father. He dropped out of college. Mr Rove's mother committed suicide 12 years later.

By this point, the myth was made. Aged just 22, Mr Rove became a footnote in the Watergate scandal when The Washington Post reported that he was among a group Republican strategists who were travelling the country, advising candidates on the subtler, rougher elements of the local campaign.

He had first turned heads as a 19-year-old volunteer in 1970, when he was accused of stealing Democratic stationery to invite a group of homeless people and alcoholics to an opponent's fundraiser, and over the next 35 years Mr Rove's rivals were hit by a torrent of bad luck and unfortunate rumours, his reputation growing all the time.

Masterminding President Bush's two election victories and the simultaneous expansion of the Republicans' control of the three branches of the US Government from 2000 to 2004, earned Mr Rove the nicknames "Bush's brain" and "the architect" but it was another sobriquet, "turd blossom", that may last the longest. The process by which a flower grows from a cowpat seemed to suit Mr Rove and he never objected to the name.

As James Moore and Wayne Slater wrote in their biography, Bush’s Brain: How Karl Rove Made George W. Bush Presidential, his political career produced a series of “unexpected, campaign-shaping events” that were difficult to trace, even harder to verify, but definitely smelly. “It became his motif. There is no crime, just a victim. Evidence is gone before acquiring substance," they wrote.

The list is infamous. In 1994, a year after Mr Rove took charge charge of George Bush's political career in Texas, the state's Democratic Governor, Anne Richards, was rumoured to be a lesbian. Voters started receiving supposed polling phone calls — a tactic later described as "push-polling"— that asked: "Would you be more or less likely to vote for Governor Richards if you knew her staff is dominated by lesbians?"

John McCain was the next Bush opponent to come unstuck. The Vietnam veteran's "Straight Talk Express", which made him an unexpected frontrunner in the 2000 primary elections, was suddenly and mysteriously derailed by questions that he had a black child from an extra-marital affair.

Four years later, another Vietnam hero, John Kerry, was simply no match for the stream of so-called "attack ads" from a group called the Swift Boat Veterans for Truth that questioned his war record.

Despite the rough-house tactics that have accompanied his candidates' successes, many of Mr Rove's innovations were assiduously copied by his opponents: from the direct mailing of voters, to the refined science of redrawing political districts to bringing so-called "value" issues to centre of political debate.

Few disputed his personal warmth. As John Dilulio — the first Bush appointee to resign in 2001— wrote in the letter to Esquire magazine that lifted the lid on the Rove phenomenon: "Some in the press view Karl as some sort of prince of darkness; actually, he is basically a nice and good-humored man."

Over the last 18 months, Mr Rove's attempts to stay out sight were countered by an equally vigorous campaign, mounted at times by his Democrat opponents, at others by federal prosecutors, to strip him of his mystery.

The Jack Abramoff affair. The mid-term elections defeat. The CIA spy leak. The firing of federal prosecutors. Mr Rove's name was dragged into a series of Republican scandals, but his precise role and responsibilities never revealed — and not without people trying to find out.

He was questioned in front of a grand jury five times before prosecutors decided not to charge him in the CIA leak case. During the trial of Lewis "Scooter" Libby, Vice President Dick Cheney's chief of staff, who was eventually jailed for obstructing the investigaton, lawyers tried to pin the blame on Mr Rove but, as the Washington Post reported, "his job was never fully explained. His influence was never clearly defined".

It was the same in the attempt to get to the bottom of firing of eight federal prosecutors by the Bush administration last year. As the inconsistent US Attorney General, Alberto Gonzales, has been dragged back and forth at congressional hearings, the Democrats have sought the head of Mr Rove. When he was ordered to appear before the Judiciary Committee last month, Mr Bush intervened to block his testimony.

During the Jack Abramoff affair, the disgraced lobbyist claimed to have the ear of Mr Rove — his assistant, after all, had once been Mr Rove's — but nothing was ever proved and Abramoff was imprisoned for six years.

"I just think it's time," he said, explaining his decision to step down now to spend more time with his second wife, Darby, and son. By leaving now, Mr Rove leaves on his own terms, the mystery intact.

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