It was a crowded primary field and Tony Evers, running for governor, was eager to win the support of officials gathered at a Wisconsin state Democratic party meeting, so the candidate did all the usual things: he read the room, he shook hands, he networked.

The digital fence enabled Evers' team to push ads onto the iPhones and Androids of all those attending the meeting. Not only that, but because the technology pulled the unique identification numbers off the phones, a data broker could use the digital signatures to follow the devices home. Once there, the campaign could use so-called cross-device tracking technology to find associated laptops, desktops and other devices to push even more ads.

Welcome to the new frontier of campaign tech — a loosely regulated world in which simply downloading a weather app or game, connecting to Wi-Fi at a coffee shop or powering up a home router can allow a data broker to monitor your movements with ease, then compile the location information and sell it to a political candidate who can use it to surround you with messages.