The relevant question on Arlen Specter's switch to the Democratic Party is not why he did it, but instead why every Republican Senator hasn't joined him. Switching is a pretty good deal:
You get a promise of no primary challenges, and overall campaign support, from the entire Democratic leadership, including President Obama. By contrast, good luck avoiding any Club for Growth primary challenges, or having the support you receive from the Republican leadership coming anywhere close to matching what President Obama can offer.
Given all of this, if I am a Republican Senator, it would seem that becoming a Democrat is a no-brainer. I get more power, an easier re-election campaign, and it costs me nothing whatsoever.
Harry Reid should offer the Specter deal to every Republican Senator. Then, with a 100 Democrat Senate, I'm sure he will be able to pass whatever legislation he wants.