You forget that McCain has presidential ambitions and will be FORCED to support a filibuster if the Democrats break their word. Think ahead a couple of moves.
This is a "memorandum of understanding", whatever that means. I don't think it has the weight of law or even a Senate rule. It can be renounced by any of the parties if they perceive that the spirit of the agreement has been violated.
Am I wrong??
McPain can't get enough Republican support to become the nomineee. Now, maybe if he runs as a RAT, he may have a chance.