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By Lois Beckett, ProPublica 

If you’re a registered voter and surf the web, one of the sites you visit has almost certainly placed a tiny piece of data on your computer flagging your political preferences. That piece of data, called a cookie, marks you as a Democrat or Republican, when you last voted, and what contributions you’ve made. It also can include factors like your estimated income, what you do for a living, and what you’ve bought at the local mall.