She's back. The woman who famously likened herself to a pitbull with lipstick returns to the centre of the political stage this week.

Sarah Palin publishes her memoir, entitled Going Rogue, today and massive success is already assured. The book has been near the top of online retailer Amazon.com's best-seller list for weeks on the strength of pre-orders alone. Its publisher, HarperCollins, paid Palin $1.25m just for signing up with them. The contents of the book are a closely guarded secret, but its launch will be accompanied by a huge publicity drive. A tour has been arranged, beginning in Michigan tomorrow. It will take Palin to 12 other cities in six days.

Several high-profile media appearances have been lined up. The biggest of them all is already in the can. Last week, Palin recorded a TV interview with Oprah Winfrey. They seem an odd pairing, given Winfrey's overt support for Barack Obama throughout last year's election, but the encounter apparently went smoothly. It was broadcast in the US last night.

Strange though it seems now, only 15 months ago Palin was a virtual unknown. Then, in August 2008, John McCain announced that the self-proclaimed 'hockey mom' would be his nominee for the vice-presidency.

Philip Gourevitch later wrote in the New Yorker that she "became the human cannonball of the presidential campaign and blasted into overlapping orbits of political and tabloid celebrity".

She continues to straddle those worlds in a way that is true of no other contemporary politician, save for Obama himself. But whereas Obama can seem to have stepped from a script of the TV drama The West Wing - all elevated rhetoric and noble intentions - Palin appears to be starring in a particularly lurid soap opera.

The most notorious example comes in the shape of Levi Johnston, the young man who was once set to become her son-in-law. Within days of being chosen by McCain last year, Palin was obliged to reveal that her teenage daughter, Bristol, was pregnant with Johnston's child.

The fervently pro-family Palin tried to counter any embarrassment by complaining about media intrusion into her family life. Bristol duly gave birth to a son, Tripp, last December.

Her relationship with Johnston crumbled shortly afterwards. He has gone on to become a persistent thorn in Sarah's side.

In the October issue of Vanity Fair, Johnston unleashed a stream of allegations: that Palin and her husband, Todd, often discussed divorce and slept in separate bedrooms; that she had wanted to keep her daughter's pregnancy a secret and adopt the child as her own; and, perhaps most damagingly, that she semi-jokingly referred to her youngest child, Trig, who was born with Down Syndrome, as "the retarded baby".

When Johnston repeated the last allegation in a CBS TV interview last month, Palin responded: "Even the thought that anyone would refer to Trig by any disparaging name is sickening and sad."

Johnston is back in the headlines for striking a deal that will see him appear naked in Playgirl magazine. "Those who would sell their body for money reflect a desperate need for attention," Palin responded.

There have been plenty of other bizarre moments for Palin in recent months, including a spat with chat show host David Letterman after he made crude jokes about Palin's appearance (he referred to "her slutty flight attendant look") and implied promiscuity on the part of her daughter.

The strangest of all, however, was the manner of her resignation as governor of Alaska. The announcement came so suddenly that Palin's main Press aide was among those caught on the hop: she was more than 4,000 miles away in New York.

The setting of the announcement - an Alaska lakeside, with only a tiny crowd in attendance and geese honking in the background - added to the air of oddness.

Then there was Palin's speech: a meandering 17-minute oration that was thick with her characteristic mangling of syntax ("And with this success, I am proud to take credit"), yet devoid of any convincing rationale for her decision.

The more inexplicable elements of Palin's story can create the impression that she is a semi-comic character. In fact, she is a lot more serious - and perhaps more dangerous - than that.

Her allegations that Obama was associated with subversives injected a poisonous tone into last year's campaign. Before long, Palin supporters were heard shouting "Kill him!" at her rallies.

More recently, she has helped to nudge dark conspiracies towards the political mainstream. She alleged earlier this summer that Obama's plan for health care reform would lead to the creation of 'death panels' that would decide whether or not to treat the vulnerable. The accusation was false, of course, but Palin's decision to voice it lent it a misplaced legitimacy.

Early polls asking Republican voters about their preferred presidential candidate in 2012 have shown Palin to be a serious contender, only fractionally behind the two front-runners, Mitt Romney and Mike Huckabee.

Palin "seized the populist mantle", comments Sam Tanenhaus, an editor at the New York Times and the author of The Death of Conservatism. "Her appeal is to those who distrust the so-called elites who are directing policy and setting the tone in Obama's administration, and also the 'media elite', located mainly on the two coasts, who support it."

Though Tanenhaus remains to be convinced of her capacity to become a presidential nominee in 2012, he notes that she remains a source of excitement to the Republican Party's base: "She is something new in our politics: a woman populist unafraid to polarise and divide."

Some key Democratic strategists believe that Palin's prominence actually helps their party. When I was covering Obama's campaign last year and Palin's attacks were at their shrillest, the future president's chief strategist, David Axelrod, said: "There are a lot of voters who are disgusted with the tone of the Republican campaign. . . I think Senator McCain and Governor Palin have helped us immeasurably."

Last week, David Plouffe, who served as Obama's campaign manager, gleefully told the San Francisco Chronicle: "I think she'll be most helpful to the Democratic Party with a high profile. I hope her book tour lasts two years."

Sarah Palin's friends and enemies are united in a desire to see her stick around.

So far there are no signs that she has any intention of disappointing them.