It's a shared norm. Part of the deal is an implied promise that the other guys also ignore the big red button. Really, though... that ship had already sailed years ago leaving the cloture rule to hang on by the barest of threads. I'm half-convinced that the current Senate would have already done away with it if only they had a slightly more reliable voting margin.

IMO: cloture is a dumb rule because we already have a robust system of checks in the form of a bicameral legislature plus presidential veto. The requirement for a 2/3rd supermajority in addition to these for regular everyday business is odious and something that no other large democracy does. I'm anti-gridlock on principle alone, even if I acknowledge the absolute chaos it will probably plunge the Senate into for the next dozen years or so.