Guessing how a person might vote — and whether they might be receptive to a pitch — has long been part of the science of political marketing. Both presidential campaigns are using “retargeting” to send ads to people who visited their websites but who didn’t leave their name or e-mail address.To track those visitors even after they’ve left, the site places a small file, known as a cookie.When that person visits another site, an advertising system can send a tailored ad after detecting the cookie.That’s how the Obama campaign can send an ad to a person long after they’ve visited the Obama site, even when their mind is on something far afield from politics — like Phelps and Beard.The cookie might even indicate a user’s interests, allowing the campaign to further tailor an ad. “If you responded to a certain kind of ad, we could hit you with a similarly themed ad at another time,” said Michael Palmer, the eCampaign director for McCain.

Gathering data on all the web visits people make, the company can then present a political campaign with “buckets” of voters described as Republican or Democrat, conservative or liberal, and by what specific issues — the person — identified by a cookie on their browser, may be interested in. Similarly, Yahoo collects information about the 140 million unique monthly visitors to its sites.