Could be mistaken, but I think it's the responsibilty of the liberal 7 of the 14 to decide "Extraordinary Circumstance" to support a Dem fillibuster. I think the RINOs decide to yay or nay on the nuclear option in turn.
Who knows how it works. But, in order to invoke the constitutional option, the Republicans will need 50 votes. That means out of the gang of 7, they need 2. I know Graham has learned his lesson, but I have to wonder whether the others have. We shall see, but my bet is that McCain will cave faster than he can run to a microphone.