Political campaigns in the US track where voters go using their smartphones

Voter targeting has grown more invasive with location data that apps can transmit from cellphones

When Donald Trump took the stage last month in Fayetteville, NC, to support Republican candidate Dan Bishop in a special election, thousands of people showed up.

Mr. Bishop was seeking their support. An outside Republican group was looking for something more. It wanted their data.

Unknown to the crowd, the Committee to Defend the President, a Republican political-action committee that supports Mr. Trump, had hired a company to collect unique identification numbers from attendees’ smartphones that evening, based on location data those phones were sending to third parties. The goal was to target ads at people it could drive to