As another election season draws to a close, political campaigns have learned a lot about many voters already, including when they previously voted, how much they make, what issues matter to them, and where they get their news. But campaigns this year also know where voters have been. The location of our phones is a powerful tool that campaigns are using to laser-target our attention.

Dozens of companies stand ready to provide that data, offering new services specifically designed for political campaigns. They home in on your location using a variety of techniques, often starting with information publicly available from state voter files and then cross-referencing it with locations shared by your phone to indiscreet apps or snatched from ad networks as you surf the web.