As potential 2016 presidential candidates begin to lay the groundwork for their campaigns, Kentucky Sen. Rand Paul’s efforts are emerging as the most aggressive. As a front page story in the Journal noted, Mr. Paul has announced political hires in three states key to winning the GOP nomination, and staffers aren’t shy about referencing a future campaign headquarters and touting their expansive email lists.

As Mr. Paul continues a three-day tour through Iowa, a look at how 2016 prospective presidential candidates have been spending their money over the past year and a half shows some likely Republican candidates like Mr. Paul and Florida Sen. Marco Rubio aren’t just talking about running—they’re putting millions of dollars into the effort.