It's not just bloggers -- The Hill is speculating about the possibility of Arlen Specter joining the Democratic Party. Part of the analysis is that Pennsylvania has closed primaries so only Republicans can vote, and lots of moderate Republicans there have switched their registration to become Democrats in the past few years. What's left of the Pennsylvania GOP is more conservative than it was when Specter beat Pat Toomey 51-49 back in 2004. Toomey is running again, and it's going to be hard for Specter to beat him by talking about how conservative he is and then beat a Democrat by looking moderate.
Much simpler to just avoid the whole obstacle course and become a Democrat.