He can't. He's trapped. The Dems have him by the short and bushy. He could have safely voted against it if he'd remained a Republican. But the RATs won't allow any show of rebellion. If he wavers they can cut off his re-election funding, run another candidate in his place, whatever. He's a turncoat. His only choice would be to run again as an independent. He HAS to remain in lockstep with OHHHH BABY or he's toast.
I thought I read somewhere that in Pennsylvania, if you lose your party's primary, you can't then run as an independent...no?