You realize that he switched parties because he was going to get primary bombed because the guys who vote in the Republican Primary tend to be knuckle-dragging, unthinking conservative clods, right? The kind of people who would call someone a RINO. The kind of people who want bullshit purity tests in their party instead of reason.
So maybe you should look down if you want to see who’s feet Specter changing parties should be laid at.
Than he could have run as an independent rather than switching parties giving the other one a filibuster proof majority.
Parties change focus over time. Currently the Republicans are focused on blind, stupid obstructionism no matter the cost. Perhaps the Democrats seemed like more of a fit after the childish tantrums that have become the staple of the right.
Also, the filibuster-proof majority assumes that every Democrat walks lockstep with every other. But there are a range of Democrats in congress. Very liberal to fairly conservative. Getting those people to vote as one is near impossible.
However, the Republicans have such a small kernel of power now, they have spun-off their moderates and only have blind, stupid, ideologues perfectly willing to outright lie left in their caucus. Democrats didn’t have a filibuster-proof majority against such unified ideological fervor. The Republicans are currently of one mind, and that mind, sadly is a fool.
Also, then. 
You realize, of course, that a senator can vote however they want on, say, a filibuster vote without being a member of a specific party–or for that matter that there were many Dems who forced changes to bills because they were holding out on giving their filibuster vote?
The two things have nothing to do with each other. And how does that relate to “spineless” in either case?
He only does what he can do to politically survive.
Changing your politics to stay in office might be considered spineless, but changing your party is not. Some pols and politically engaged non-pols have changed parties claiming quite plausibly that their politics have remained the same but the party has changed around them. I state no opinion as to whether that is true in Specter’s case, but the possibility is essential to bear in mind.
Meh! The man stuck by his principles in the face of an increasingly insular Republican party, and only switched parties when his own party went from being very happy with him in the 80s and 90s to threatening to run someone against him in the primaries now.
Look, man, I LIVE in PA, and I have for over twice as long as you’ve been alive. Specter’s one of the best politicians we collectively have regardless of the little letter beside his name.
If he went Independent I would bet real money that he’d get re-elected anyway, just like Lieberman–Specter’s just being somewhat more honest about it by putting up the D.
Look at it this way–he’s got to beat Joe Sestak in the primary now, and then still has to beat Pat Toomey (who’s a complete and utter jerk) in the main election.