The presidential election officially is decided by the states through the Electoral College. But to fully understand the race’s dynamics, it also helps to look one level down – at counties.

That’s what U.S. News has done through a series examining 15 battleground counties that could prove key in this year’s race for the White House.

Eleven of the 15 counties are located in the seven presidential swing states – Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin – on the logic that a county’s electoral leverage is greater when it sits in the most competitive states. Three of the remaining four counties are in large states that are on the periphery of competitiveness – Florida and Texas – and one county is in a category all its own because it sits in Nebraska, which allocates some of its electoral votes by congressional district.