Specter's public unwillingness to change:
After vowing not to support cloture on the Employee Free Choice Act in his first day as a Democrat, and then voting against the Democratic budget in his second day as a Democrat, Arlen Specter has now, in his third day as a Democrat, voted against the housing bankruptcy reform legislation known as "cramdown."
He also continues to oppose a key Obama nominee, and is against a public option for health care reform. No Democrat, not even Evan Bayh or Ben Nelson, has acted against the Obama administration this often, and certainly not on this number of major pieces of legislation.
If Sestak doesn't challenge Specter at this point, it will likely be because he decides it is not viable, rather than because Specter actually changes his positions. In that regard, Sestak would probably receive support from the the AFL-CIO, SEIU, and much of the netroots. However, he would still be going up against a massive array of Democratic Party leaders, including President Obama. Also, Specter currently has a sky-high, 80%+ approval rating among Pennsylvania Democrats.
It is a tough call for Sestak, but at the very least he is doing progressives a service by keeping his threat of a primary challenge so open and public. If Specter actually is going to start acting like a more reliable Democrat, there is a much better chance that he will do so with the sword of Sestak hanging over his head, then with the primary path cleared for him. As he revealed in his partisan switch, Specter clearly wavers when under threat of defeat.