"Consider Specter’s most significant votes over the last eight years, ones cast in favor of such definitive right-wing measures as: the war on Iraq, the Military Commissions Act, Patriot Act renewal, confirmation of virtually every controversial Bush appointee, retroactive telecom immunity, warrantless eavesdropping expansions, and Bush tax cuts (several times).  Time and again during the Bush era, Specter stood with Republicans on the most controversial and consequential issues.

"(2) Democrats will understandably celebrate today’s announcement, but beyond the questions of raw political power, it is mystifying why they would want to build their majority by embracing politicians who reject most of their ostensible views.

"Reports today suggest that Democratic officials promised Specter that the party establishment would support him, rather than a real Democrat, in a primary. If true, few events more vividly illustrate the complete lack of core beliefs of Democratic leaders, as well as the rapidly diminishing differences between the parties. Why would Democrats want a full-blooded Republican representing them in the blue state of Pennsylvania? Specter is highly likely to reprise the Joe Lieberman role for Democrats: a “Democrat” who leads the way in criticizing and blocking Democratic initiatives, forcing the party still further towards Republican policies.

"(3)  Arlen Specter is one of the worst, most soul-less, most belief-free individuals in politics.  The moment most vividly illustrating what Specter is:  prior to the vote on the Military Commissions Act of 2006, he went to the floor of the Senate and said what the bill "seeks to do is set back basic rights by some 900 years" and is "patently unconstitutional on its face."  He then proceeded to vote YES on the bill's passage."

From Atrios: "I'm obviously not very optimistic that this will lead to good. I've lived under the wanktitude of Specter long enough to know that he rarely actually does anything positive. As Harry Reid said to a small group of bloggers last year in Denver, (quote from memory) "Arlen Specter's with us except when we need him." Question now is whether Specter will be with them when they do need him.

"On the plus side, Senate staffers inform me that Republicans in the Senate are visibly in agony right now. So at least we have that!"

From Chris Bowers at Open Left: "So, here is how I understand things: 

1. We get no new votes on legislation from Specter 
   
2. Democrats are given no opportunity to challenge Specter in either the primary or general election, thereby locking all of his bad votes into place even though he is in a blue state.

And from dday at Hullabaloo: "The recent history of these things is that the party-switcher ends up landing in the mainstream of his new party. But in Specter's case, with the Democratic Party already coalescing around him and asking for little in return, that may not be so. In his press conference, Specter said he would retain seniority inside the Democratic Party and would therefore be likely to possibly earn a committee chairmanship in the next Congress. Harry Reid seems to have given up way too much to earn Specter's place in the caucus, considering that he was a goner if they refused."

Over at DownWithTyranny: "Specter made it clear he would just be changing party affiliation, not his Republican mindset:

My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats that I have been for the Republicans. Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change.