Why Did Specter Defect?

Arlen Spector who was a long time Republican Senator from Pennsylvania, became a Democrat. Why? Is it only because he couldn't win reelection as a Republican against Toomey? Is it only survival or ideological? Is it a cold, clinical calculation or thoughtful progression toward the obvious? Real question: why is the GOP is such disarray? They are falling out with each other. They appear petty and petulant. They are no longer a national party. One thing for sure, this is only temporary. Politics creates unexpected opportunities. What do you say?