Chris and David already expressed why those who call themselves "progressive" over "Democrat" (and even many who call themselves "Democrat") are concerned about Specter potentially getting a free ride with his a party-switch today.
Another element of this story, from Politico:
The move also raises the stakes for the resolution of the Minnesota Senate race and may tempt Republicans to drag that fight on further.
Norm Coleman's latest court challenge won't even have paperwork submitted until June, then comes the hearing, then the verdit, then the next appeal.
It's becoming increasingly clear that it'll take a grassroots uprising to make Coleman go away -- and Specter's announcement makes the urgency all the greater. Regardless of whether we want Specter to win re-election in 2010, we now have a bigger opportunity to pass some big ideas in Congress -- if we can seat Franken.
If you haven't already, now's the time to consider joining thousands of others on the "Dollar a Day to Make Norm Go Away" campaign -- giving $1 each day Coleman refuses to concede to help progressive candidates defeat Republicans in 2010. Together, the grassroots can change the incentives for Coleman's DC funders and dry up the only support that is keeping his ridiculous string of court challenges alive.