Th first point, as others have already pointed out, is he may finally be making official what his true status has been for years - a Liberal in hiding, pretending to be a “moderate” Republican; which he never was.
The second point has nothing to do with the Senate, per se. It has to do with that fact that Toomey is way ahead of him in the polls for the GOP primary for his seat. Sensing defeat, he hopes to do better, FOR HIMSELF, as a Democrat.
The third point is that we can expect a lot of moral blather from him, telling lies, about how “the GOP abandoned him”.
The fourth point IS the senate, and its role in judges, spending & treaties. He’s going to NOW run, in Pennsylvania, as the unashamed Liberal he is, and set a course in the Senate to convince Liberal Pennsylvanian’s of that.
True to what Arlen has always been, he’s putting himself, and no particular principal, first.
Exactly!