The political world is fixated on whether this year's elections will deliver an epic rebuke of President Barack Obama and his party. If that happens, it could end up costing Democrats congressional seats for a decade to come.

Some of the most important contests this fall will be way down the ballot in communities like Portsmouth, Ohio and West Lafayette, Ind., and neighborhoods like Brushy Creek in Round Rock, Texas, and Murrysville Township in Westmoreland County, Pa. These are state legislative races that will determine...