IF THE race card does get played in November's US election - and with Barack Hussein Obama II vying to be America's first black president, it's hard to see how it won't - then it's as likely to be dealt from Sarah Silverman's deck as from the hand of Obama's Republican rivals. Although the 38-year-old Jewish joker is relatively unknown in the UK, one British newspaper recently described her as "the world's hottest, most controversial comedian". In her homeland, she is feted and reviled in equal measure. To some she is a racist - "Everybody blames the Jews for killing Christ," goes one oft-quoted routine, "I'm one of the few people that believe it was the blacks". To others, she is a sophisticated performer whose jokes about race, rape and abortion make her heir to the legacy of Lenny Bruce and Andy Kaufman. "I was raped by a doctor, which is so bittersweet for a Jewish girl," goes another jaw-droppingly offensive gag. There are many more.

Like Bruce, then, Silverman inhabits that tiny sliver of space where the sayable and the unsayable intersect. You could call it the discomfort zone. She occasionally strays out of it, such as when she used the racial slur "Chinks" on the chat show Late Night With Conan O'Brien, but mostly she negotiates it brilliantly.

Her career began on Saturday Night Live, the famous American comedy nursery whose alumni include Kaufman, Bill Murray, Dan Ackroyd, Eddie Murphy, John Belushi and Mike Myers, and today she has her own show, The Sarah Silverman Program, on the Comedy Central channel.

But while Silverman is the highest profile female comedian working in the US today, she is only one of many. Behind her is a wave of female writers and stand-ups who are ignoring the more familiar subjects of distaff humour and tackling the sorts of cultural and political issues that were previously the province of male comedians.

Like Silverman, black stand-up Tracey Ashley riffs on race. In one sketch she trades insults with an Asian man who objects to how she has parked her car. In another, she ruminates on what life would be like under a black president. In a third, what would have happened if Hillary Clinton had won the nomination and the election and drafted into her cabinet every other political wife who had been cheated on by her husband. "Osama'd be like, I'm not messin' with them bitches'," Ashley scoffs. Political it is, politically correct it isn't.

They're not the first funny women to try to tackle political subjects, of course. As recently as 2004 the veteran comedian Sandra Bernhard could be found railing against the fact that women weren't "allowed" to do social commentary. "I think it's the last gasp of the white male, the patriarchy that excludes us," she told Ms Magazine. "They are digging their heels in harder than ever." But this is the first generation to be succesful at it and accepted for it. So nearly half a decade on from Bernhard's complaints there are, finally, some scuff marks where those dug-in masculine heels once were. The women may not have won the ensuing dust-up but at least they share the field with the men. Journalist Christopher Hitchens still refuses to cede ground - last year, he wrote an essay in Vanity Fair magazine entitled Why Women Aren't Funny. But he is in the minority. Women, it is agreed, are funny.

Of course, women have always been funny. The one-liners of Dorothy Parker and Mae West are legendary and in the context of American television women like Lucille Ball, Mary Tyler Moore and Carol Burnett have won generations of admirers. But what has changed is this: there has never before been a time when so many women have had so much control over the tenor and slant of their humour, so many opportunities to air it, and so much appreciation for it.

Marking this seismic shift in the power balance of US comedy (and implicitly nullifying Hitchens's arguments), Vanity Fair recently commissioned photographer Annie Leibovitz to make portraits of this distaff laugh brigade. Leibovitz chose 12 women in all, prime among them Silverman, Bernhard and Tina Fey, writer and star of TV series 30 Rock and the first woman to head the Saturday Night Live writing team. Silverman made the cover alongside Fey and fellow Saturday Night Live member Amy Poehler, all of them dressed like Greek goddesses.

Inside, Leibovitz turned Silverman into a debauched rock chick - think Amy Winehouse-meets-Sandra Bullock - and dressed Fey in a black cocktail dress to photograph her in the back of a limousine alongside Kristen Wiig and Maya Rudolph, one current and one former member of the Saturday Night Live cast. They looked like Hollywood starlets on their way to a post-Oscars party. Possibly even the famous Vanity Fair post-Oscars party. The whole spread reeked of power, sass and confidence.

For Alessandra Stanley, television critic with the New York Times and author of the article which ran alongside Leibovitz's portraits, the real sea change has been this sense of confidence and with it the new-found freedom to attack bigger subjects than cellulite or the menopause.

"There has been an epochal change even from 20 years ago, when female stand-up comics mostly complained about the female condition," she wrote. Previously, there were only two schools of what she called "acceptable" female humour. They were "feline self-derision or macho-feminist ferocity" and they were best summed up by the comedy of Joan Rivers and Roseanne Barr. Today, it's different. Today, anything goes.

Although Rivers is a heroine to many of the younger generation of female comics, it's true that her act conforms to a model of humour that now seems outmoded. She remains catty and poisonously funny, of course, but her self-deprecating persona jars with the ones inhabited by younger female comics.

"My best birth control now is to leave the lights on," is a celebrated Rivers quip. But contrast that with young US stand-up Jen Kirkman's routine about masturbation. "How long do women have to live on this earth before they can talk about the things guys talk about?" says Kirkman, as she introduces a rambling (and very funny) complaint about how co-habiting with her boyfriend has curtailed her opportunities for self-love.

And how does Barr, who once said, "The quickest way to a man's heart is through his chest", compare to a stand-up like Lisa Lampanelli, who appears on-stage wearing a purple dress and Jimmy Choo shoes and who makes risque jokes about gays, blacks, Jews and fisting? Like a gramophone compares to an iPod, that's how.

So what does all this tell us about America in 2008? Scotland will have a chance to find out when a number of these female American comics make the trip across the Atlantic to appear at the Edinburgh Fringe. Among them are Seinfeld star Carol Leifer and hotly-tipped stand-ups Morgan Murphy and Kristen Schaal.

Leifer is fiercely pro-abortion and in 2006 was one of 5000 women who signed a We Had Abortions petition published in Ms Magazine. Expect a few broadsides against the religious right in her show.

Morgan Murphy, meanwhile, is a regular on Jimmy Kimmel Live!, a late-night chat show hosted by Sarah Silverman's boyfriend. Murphy, who used to write for Kimmel, looks like Crystal Tips and skits about acting in a bestiality porn film, being stalked, and how she tried to gas herself in an electric car.

The humour of Kristen Schaal, who features in the HBO series Flight Of The Conchords, is more surreal but no less dark. In a filmed skit posted on her MySpace page she smokes seductively against a driving funk soundtrack - and then vomits blood and tar all over the camera. Schaal is also a contributor to The Daily Show, the Emmy Award-winning political satire show hosted by Jon Stewart.

Finally and fittingly, Joan Rivers is also coming to Edinburgh. Now 75, she will perform her one-woman play Work In Progress By A Life In Progress on the Fringe ahead of a transfer to London's West End. She still has the power to shock, however, as the presenters of ITV's Loose Women found out to their cost earlier this month when she loosed one of her verbal Exocets at actor Russell Crowe. "Get ready to bleep," she smirked, before calling the Australian "a piece of f***ing shit."

Unfortunately, the programme was live. Rivers apologised the next day on the Richard And Judy show. "I was wrong, I was wrong," she said, looking anything but contrite. "I meant Mel Gibson." Boom boom, as the actress said to the actress.