Greg Archuleta lives in Golden, Colorado, where he worked for the Coors brewery for 34 years until he retired in 1999. Archuleta, who is 73, volunteers for the Democratic Party in the larger Jefferson County area, 778 square miles of suburbs just west of Denver that holds half a million people. On a recent Saturday drive, Archuleta was worried. For the past few months, he’s been asking property owners with backyards facing the highway if they would hang giant signs for President Barack Obama and the local congressman, Democrat Ed Perlutter, who’s in a tough battle for re-election. Now, some of the Obama signs had come down; more and more signs for Mitt Romney were up. Archuleta drove between shopping malls and new condos and subdivisions, investigating the grassy tracts between road and neighborhood. “There it is!” he’d shout when he spotted one.

There’s a good reason Archuleta is counting the signs. Jefferson County is one of a handful of districts in the nation with the power to swing a swing state, and thereby determine the outcome of the national election. Colorado’s District 7 is a swing district by design. A judge created it after the 2000 census gave Colorado a new seat in Congress and the state legislature couldn’t agree on its boundaries. The judge made it a snapshot of the state economically and politically: The population was divided into even thirds of Democrats, Republicans, and independents. In the decade that followed, the district began to tilt Democratic until it was redrawn after the 2010 census, after which it split some areas with neighboring District 6, which leaned more Republican, making both more competitive.