Digital advertising networks and large web companies like Yahoo and Microsoft are using web behaviour — which news articles people read, which blogs they visit or what search terms they enter — to target voters who may be sympathetic to a certain cause. Using a method known as “sentiment detection”, some companies even boast of telling whether the blog you go to is for or against the Iraq war.

“During a get-out-the-vote drive, you don’t want to get out the wrong vote,” said Diane Rinaldo, political advertising director at Yahoo. The advertising techniques, known as “behavioural targeting” and “retargeting”, have raised alarms from some privacy advocates, who say no one should unwittingly have their political leanings analysed as they use the web, or be tracked for the delivery of political ads.“The web has been hailed for creating new opportunities for political expression, but there is this dark underside to it,” said Jeffrey Chester with the Center for Digital Democracy. Advocates of the practice, say its use in the political world is comparable to traditional direct-mail campaign practices. By contrast, most online targeting is directed to a web browser, and the name and home address of the target is unnecessary.

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