Two weeks out from Election Day, Republican Pat Toomey appears poised to lead a Republican surge in Pennsylvania in a Senate race that will test just how deeply the state's "blue" roots run.

The battle between Mr. Toomey, a former congressman, and Rep. Joe Sestak, a Democrat, is part of a larger story of this year's congressional campaigns: Democrats find they are devoting money and manpower to save Senate seats in areas that were once considered their political backyards. In Pennsylvania, Senate Democrats' campaign committee has spent more than it has on any other race, and Mr. Sestak has been aided by visits from President Obama and former President Bill Clinton...