The U.S. unemployment rate fell sharply in September to its lowest level since January 2009, suggesting that summer job growth was stronger than previously thought and providing new fodder for a presidential race that has focused on competing views of the nation's economic health.

Data released Friday portrayed a labor market that has perked up a bit since the spring but is still growing modestly. The unemployment rate slid to 7.8%, the Labor Department said, falling below 8% for the first time since President Barack Obama's...