A few days after the so-called Biscuitgate affair — in which Gordon Brown failed to tell the users of Mumsnet whether he preferred Jaffa Cakes or Kit Kats — a packet of Downing Street own brand chocolate chip cookies arrived at the parenting website’s Kentish Town office in north London. The expiry date was November 2009 and it was October 31 but it was the thought that counted. “I hope you like the biscuits,” read a (correctly spelt) handwritten note.

Not long afterwards, Justine Roberts, co-founder of Mumsnet, was invited to No 10. The Prime Minister had loved his mauling by the mothers, she was told, he’d be delighted to have regular webchats and other ministers would be told to do the same. Labour would also like to test ideas and make policy announcements through the website ahead of the election.

It’s not that the Prime Minister is a sucker for punishment. Even Tony Blair, who got his thrills from the masochism strategy, didn’t rush back to the Women’s Institute after being slow-handclapped by the lemon curd ladlers. This is about political strategy. All the parties have decided that women are the key to electoral success, that the family will be a critical issue when the country next goes to the polls and that the internet is a vital campaign tool. Just as Worcester Woman was the symbolic swing voter in 1997, and “soccer moms” were Bill Clinton’s target during the 1996 US presidential campaign, so next year’s poll will be the Mumsnet election.

On Thursday David Cameron is trundling up to Mumsnet Towers, bearing oatcakes, for a web chat with the site’s 850,000 users. “We want to find ways of boosting the role of non-state collective action and Mumsnet is a brilliant example,” says a Tory strategist. “We’re really keen to support them.”

Ed Miliband has agreed to do a webchat in the run-up to the Copenhagen summit next month. Ed Balls recently began a discussion about changing the school starting age. Harriet Harman sought Mumsnet users’ views about women in the recession. And when swine flu arrived, Andy Burnham, the new Health Secretary, got straight on to the site to spread the word about what parents should do.

It’s no coincidence that Labour is preparing to abandon a plan to scrap childcare vouchers: Mumsnet has been leading the campaign to keep them. Deborah Mattinson, Mr Brown’s pollster, says: “Mumsnet is totemic of the modern mothers who will be the key political battleground at the next election.”

In the past five years the family has become an online phenomenon. There are 751 members of British Mummy Bloggers and there is a monthly Tots 100 index of parenting blogs. Uma Thurman’s latest film, Motherhood, is about a “mummy blogger” in New York.

For the politicians, Mumsnet is something more — it’s a virtual version of the school gate, a place where women exchange ideas about everything from nativity play costumes to health policy in a context where they are likely to listen to what is said because they are part of an online community. It is also a perfectly balanced, politically interested national cohort of swing voters — 17 per cent vote Labour, 16 per cent Conservative, 16 per cent Liberal Democrat, 14 per cent others (mostly Greens) and 37 per cent either don’t know or won’t say what their political views are. There have been several discussions asking questions along the lines of: “Can I vote Tory for the first time now that David Cameron is in charge?” Last week, though, the mums were overwhelmingly supportive of Mr Brown over his condolence letter.

“It’s a site for parents but the most popular topics are not traditional parenting topics,” says Justine Roberts. “They’re interested in City bonuses as well as nappies. We’re a bellwether for what women are thinking.”

And of course all the political parties are trying to woo female voters in the run-up to polling day. It was the defection of women from the Tories that handed power to Labour in 1997, and they were responsible for the party’s re-election in 2001. In 2005 the “let-down ladies” almost turned on Labour — until they looked at Michael Howard and ran back to Tony Blair. Now, one Downing Street source admits: “We’ve lost the women.”

Female voters do not warm to Mr Brown: Biscuitgate resonated because it reflected a wider truth about the Prime Minister’s inability to communicate on a human level. (If Mumsnet were in charge, Labour would be led by the Miliband brothers.) But many are still wavering over how to vote. Once bitten twice shy, the women who fell head over heels in love with Mr Blair in 1997 are not going to allow themselves to be charmed so easily by another smooth public schoolboy.

A Cabinet minister says the next election campaign is going to be based on three F-words: future, fightback and family. Labour is planning manifesto pledges to increase paternity leave, allow greater flexibility at work and give more help to those caring for elderly parents. The Tories are also preparing to pitch to the Mumsnet vote with an increase in parental leave as well as 2,700 health visitors.

For the first time the leaders of all three main political parties have young children. David Cameron shares the school run with his education spokesman, Michael Gove. Steve Hilton, the Conservatives’ chief strategist, leaves at 5pm every night to bath his young son. Sarah Brown used to joke that it was great being married to Gordon because when one of the children woke at 4am he was already up tapping away at his computer so could put him back to bed. My last interview with Nick Clegg was interrupted by half a dozen phone calls about history homework.

The electorate will be choosing between three dads at the next election — which may be why the political parties have realised that they have to appeal to the mums.

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