WASHINGTON - A Democratic congressman has decided to abandon his suburban Philadelphia seat to run for the Senate seat now held by Republican Sen. Arlen Specter, The Associated Press has learned. The decision by Rep. Joseph Hoeffel probably will pit the three-term Democrat against the winner of a bruising Republican primary next April between the moderate Specter and a challenger who already has announced, conservative Rep. Pat Toomey , R-Pa. Hoeffel's formal announcement will come in the next several weeks, a source with knowledge of his decision said on condition of anonymity. His spokesman, Frank X. Custer, would not confirm...