How politicians target you: 3,000 data points on every voter, including your phone number

The campaign messages are coming fast and furious now.

“Hi Geoffrey, I'm Jess w/ People's Action,” reads one. “Voter Alert for Geoffrey Fowler!” says another.

And the weirdest: “It’s Jonathan Del Arco, Hugh the Borg on Star Trek … Join a grass roots fundraiser with 19 cast members!”

Perhaps your text messages, Facebook feed or mailbox have also exploded with eerily personal political ads. Ever wonder: How’d they find me? I, for one, didn’t pass my digits to a campaign — much less tell them (or Hugh the Borg) I’m...