It's becoming increasingly apparent that Arlen Specter is not a man who places a high premium on principle. (My colleague Jonathan Chait, to his credit, perceived this earlier than most people.) But the nice thing about nakedly opportunistic politicians is that their ever-shifting positions are a leading indicator of changing political currents. 

As such, Specter's sudden decision to consider a public insurance option as part of reform would seem to suggest political momentum behind health care reform is building--and that efforts by groups like Health Care for America Now are having an impact.

Update: And Specter isn't alone. Ryan Grim has details over at the Huffington Post. 

--Jonathan Cohn