Once a decade after the census, states redraw their congressional and state legislative lines to ensure equal population across districts. Those maps can and often have been drawn to make it easier for one party or the other to win seats, a practice known as gerrymandering. Sweeping Republican victories in statehouses across the country in 2010 helped solidify GOP control of Congress for years. In Pennsylvania, the victories yielded a congressional map that was considered one of the most extreme partisan gerrymanders ever. The Pennsylvania Supreme Court last year threw out those maps and redrew new ones itself.