To begin with, let’s get a snip of this morning’s op-ed up on National Review: 
Arlen Specter belongs to a type familiar to Congress: the time-serving hack devoid of any principle save arrogance. He has spent three decades in the Senate but is associated with no great cause, no prescient warning, no landmark legislation. Yet he imagines that the Senate needs his wisdom and judgment for a sixth term. He joined the Republican party out of expediency in the 1960s, and leaves it out of expediency this week.
Indeed. At the end of the day,what we have here is the second in a line of what will be many ‘victims’ of what are now being called the Tea Party protests. The first, I think, was John McCain.
Now, you’re going to be hearing, over the next weeks and months between now and the mid-term elections, how the supposed GOP swing to the extreme right has cost the Republicans the 2008 election. These charges have come from such as Lindsay Graham and Ramesh Ponnuru, among others, and of course from staunch Democrats, who it would appear are simply pulling themselves up on any available handhold. Specter, in particular blames that factor on his leaving the party. But it’s not so. In truth, the movement of the Republican party for the time that Specter has been in office, has been to the left… The exception… Reagan.. being their wildest success. That leftward march since Reagan has damaged the party, and the country and culminated in the GOP losses in 2008. The Tea Parties have been a reaction to that starboard movement, and one over-long in coming. The shut-out of Specter is merely one step on the path back to GOP principles… and in my view, an encouraging one, as I mentioned in my recent Pajamas Media article. 
But for all his protests about how the party left him… thus invoking Reagan, interestingly, who used that line to explain why he abandoned the Democrats in the 60’s to follow the call of the then-conservative Goldwater… the fact is that even moderates have been deserting Specter for quite a while, now. The most recent one I’ve seen suggests that Specter was only leading among Republican moderates by around 8 points.
The fact is, that the party that has been narrowing, these last few years, and particularly the last couple of years, is the Democrat party. As Jennifer Rubin points up yesterday, at Pajamas Media: 
With market capitalism under assault and polling showing voters quite concerned about spending, debt, and bailouts, you’d think Republicans could find a message which resonates with a wide audience.
And so they have. Thing is, the Republican rank and file recognizes in a way the party leadership is only starting to understand that the first task is to eliminate those who don’t share those concerns. And Specter, I’m afraid is one such. (I’ll take up my differences with Jennifer’s conclusions in another article)
So, Specter, instead of fighting for the principles he claimed to believe in, jumps ship when he sees his political future coming to a sticky end, and now argues against those principles. Apparently, other than his own political future, Specter HAS no principles. As NR puts it:
We are not sure why self-respecting liberals would vote to have Specter as their representative. Most Democrats believe the country is with them and Pennsylvania even more so; why not have a true believer in the seat?
That’s exactly the issue that we’re going to see played out in PA, over the next several months. Already, he’s got two running against him in the primary, who have more popular support.
Now, lest anyone think that because the state is heavily Dem, Toomey, Specter’s Republican challenger, has no chance at Beating Specter, I’ll point out that Toomey has repeatedly won in heavily Democrat Beaver County, and by a wider margin than Specter ever did.
There is, however, another layer to all of this: The large number of Democrats who are none too happy with where their party, and thereby their country, is headed, either. By a number of reports I’ve seen some 20-25% of the people in those tea party protests were Democrats. I suspect that there are many Dems who will see Specter as part of the problem.
As the NR points out what the Democrats are taking on, here is a loose cannon.
Reporters are lazily saying that the Democrats will now have a filibuster-proof majority in the Senate, but in truth whether that majority holds will depend on the issue.
… and of course where Specter comes down on that particular issue. More particularly, if it’s in Specter’s short-term political interest to vote with his new party. A Veto-proof majority requires someone who will vote in block, and Specter has a history doing exactly the opposite if it suits his own political fortunes… which are in the end, his only concern. No worries for the people, no association to anything resembling principle will be brooked in his personal deliberation on votes before him. That aspect of Specter’s history is fairly well known, very well-noted in his voting record, and is brought into even starker contrast by this move yesterday.
For all Specter’s statements of loyalty to his new party and their agenda, Democrat voters will always recall, particularly come voting time, that he told the Republicans the same thing, in April of 2009.