The campaign, which went live this afternoon, claims that couples earning over £31,000 will lose out under a Conservative government.
The campaign is backed by a new report from the Institute for Fiscal Studies that Labour says shows that George Osborne's plans are badly flawed.
It says that research by the independent think tank was originally cited by Osborne as confirmation that his plan to cut child tax credits would only mean ending support for families on incomes over £50,000.
However, Labour says that the IFS' Green Budget report released this week spells out that in order to reach the £400m in savings that she shadow chancellor wants to achieve he would need to cut support to modest and middle income couples earning over £16,000 each.
The use of the Mumsnet website follows mums being identified as key group in the coming election. In November The Times said that the 2010 poll will be the "Mumsnet election".
Kerry McCarthy, MP Labour's media campaigns spokeswoman, said: "In the last few days we've seen the difference between the way Labour and the Tories are approaching online campaigning. Whilst the Tories are busy telling their candidates what they can and cannot say online, we're engaging with mums about the issues they care about where they come together online."
Last year Gordon Brown invited Mumsnet founder Justine Roberts to Number 10 and Tory leader David Cameron has also been courting mothers.
In January month Mumsnet hit the headlines again after hundreds of mums took part in the campaign spoofing David Cameron's airbrushed billboards.
The rise of the Mumsnet site has its roots in the US where the term mommy blogger was coined to describe the group of women who are parents in the age of social networking an blogs
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