It started, like all the best Arianna stories, at a Beverly Hills dinner party. The hostess was Candice Bergen, the Hollywood actress. The guests included a couple of television actors from the political TV series The West Wing and a real political couple from Washington: John McCain, the Republican senator, and his wife Cindy.

Also at the party, held at Bergen’s home just a few weeks after George W Bush won the 2000 presidential election, was Arianna Huffington (née Stassinopoulos), the Greek-born socialite who has become one of America’s most successful bloggers.

It was Huffington’s memories of that star-studded dinner last week that provoked one of the more improbable rows of the 2008 election campaign. On her website she had claimed that McCain, this year’s Republican candidate, known for his independent streak, had confessed to his fellow diners that he had failed to vote for Bush. Cindy, Huffington also alleged, had scribbled her husband’s name on the ballot paper.

“Totally false,” retorted McCain, who had run against Bush in the Republican primaries but bowed out after a vicious campaign of character assassination that included the anonymous and false allegation that he had fathered a black child out of wedlock.

McCain was later reconciled with Bush and has recently appeared to be on good terms with him. On her website, Huffington bluntly described the Arizona senator as “pandering in the service of ambition”. McCain, she said, “has morphed into an older and crankier version of the man he couldn’t stomach voting for in 2000”.

One of McCain’s senior advisers promptly retorted with an equally brutal assessment of Huffington: “She’s a flake and a poser and an attention-seeking diva.”

Last week the diva was revelling in all the fuss. “I know I’m right and there are others who were there who will prove it,” she told me as she sipped English tea in the lounge of the Four Seasons hotel in Manhattan. “McCain didn’t vote for Bush – and he’s not the man he’s pretending to be.”

The headline-grabbing row proved yet another feather in the cap of The Huffington Post, the startlingly successful website that features not only Huffington’s personal blog but also an increasingly formidable range of liberally inclined commentaries and news reports. Since Barack Obama and Hillary Clinton began squaring off in the Democratic primary campaign, the Post has become the most visited political site on the internet, according to Hitwise, which measures online traffic.

“It has become my whole life, other than my two daughters,” Huffington said in a soft voice that, even after 27 years in America – and a dozen more before that in Britain – retains a marked Greek accent.

The McCain row could scarcely have come at a more opportune moment for the 57-year-old author, mother and budding internet tycoon. On Thursday she published a new book, Right is Wrong, about the “far-right cowboys” and “lunatic fringe” of the Republican party, which she refers to as “the dark, mouldering, putrefied party of Bush, Cheney, Rove . . .”

Yet it was not so long ago that Huffington was a Republican supporter herself, married to a wealthy Republican congressman. So why is she now so desperate to derail the Republican election machine?

The answer may lie in another dinner party, held at the home of yet another of Huffington’s Hollywood friends: Norman Lear, the film and television producer. At this one, which took place in 2004, one of the guests was a young black politician from Illinois called Barack Obama. “It was before he was elected senator,” Huffington recalled. “I talked to him and it was clear that you were in the presence of a real leader.”

Even when Clinton declared her presidential candidacy, Huffington had no doubt about who would get her vote. “Yes, I was obviously excited by the prospect of the first woman president,” she said. But having been opposed to the Iraq war from the start, she could not countenance voting for a candidate who had supported it.

“My main criticism of Clinton is that she not only voted for the war, but never really challenged it, right up until the [election] primaries began,” she said. “To break with the past we needed a candidate who really represented change. Obama had the judgment from the beginning to be against the war.”

Huffington first came to public notice in the 1970s as the first foreign woman to be elected president of the Cambridge Union. In an early book, The Female Woman, she took critical aim at feminism and quickly became a television pundit-cum-game show personality. In London she was best known as the girlfriend of Bernard Levin, the celebrated Times columnist and critic who was 22 years her senior.

Private Eye used to call her a Greek pudding but the perfumed vision that wafted into the Four Seasons last week looked more like a minimalist dessert. Tall, slender and elegantly encased in a pencil-thin black trouser suit, she is ageing as well as you might expect for a woman who can afford the best of America’s beauty preservation products.

And she is rich, having acquired a fortune when she divorced her husband in 1997.A year later Michael Huffington, heir to a Texan oil fortune, declared himself bisexual.

Somewhere along the way Huffington shed her Greek name – “I got tired of having to spell Stassinopoulos,” she says. Meanwhile, she remains on the best of terms with her exhusband, who is an occasional contributor to her website.

“It wasn’t easy at the beginning, but we both made a huge effort for the children,” she said. “We have our daughters’ birthdays and holidays together. It has become a real friendship that I’m grateful for.” There is no new man for now: “I’m not opposed to being attached,” she added wryly.

It is tempting to conclude that she shed her Republican views along with her husband. In fact, she was never terribly convincing as an American rightwinger – her views on abortion, gun control and gay rights owed more to London than Louisiana. Which may be why she was initially so close to McCain, another independent Republican.

“I admired him, even loved him then,” she said. “A large part of the electorate hasn’t noticed McCain’s Shakespearian fall. His nobility has given way to pandering to the party’s right wing – these voters still clearly think of him as a straight shooter.”

Appalled by the way Senator John Kerry was treated by the Republican machine in 2004, Huffington is nervous that Obama may become the next victim of malevolent “rightwing messaging” and that the Republicans will find a way to wriggle back to the White House, as they did in 2004.

None of Huffington’s views would matter much were it not for the millions who now read the HuffPost, as her website has become known.

Huffington is editor-in-chief and dictates her own – occasionally racy – blogs.

On Friday she headed off to a third anniversary party for the website. She has several more stops on her book promotion tour and then has a speech to deliver in London.

No one should think of trying to dismiss her as a rich socialite. Least of all McCain: the two West Wing actors who also attended that dinner party have now confirmed Huffington’s version of events.