From the pollster's analysis:
The recent tightening of the race comes from an increase in support for Sestak, while Toomey's support remains fairly steady. Sestak's support is now at its highest level since mid-May, following a brief bounce after his Democratic Primary win over incumbent Senator Arlen Specter.
At the same time, the latest results mark the first time Toomey has reached 50% since polling of the race began in February. Prior to today's release, Toomey has earned 42% to 49% support since February, while Sestak has picked up 36% to 46% of the vote.
The TPM Poll Average gives Toomey a lead of 46.7%-43.8%.