Blue State Digital (BSD) will work with the Hope Not Hate campaign to build up grassroots support over the internet.
In particular, it is attempting to stop the BNP winning a seat in the European election and has already invited thousands of people to join the campaign online.
Hope Not Hate was founded by Searchlight, the anti-British National Party organisation.
BSD, which has offices in New York, Washington and Boston, opened a London office in December 2008.
London director Matthew McGregor said: 'BSD will be working closely with Searchlight, developing messaging, email and fundraising campaigns to help supporters organise and mobilise - and then fight back against the BNP's message of racism, intolerance and hate.'
Mark Hanson, senior consultant at Staniforth and Labour Party strategist, said: 'We are seeing a lot of groups trying or claiming to adopt Obama techniques but I am impressed by Hope Not Hate. It has grasped that building support is about reaching people who have a network of friends who will act if asked to.
'It is then crafting the appeal and transmitting the request in a powerful way as opposed to getting hold of a huge email list and broadcasting at them. BSD mastered that art in the US election and the Hope Not Hate appeal is good in that it is a simple ask and a very powerful one.'