But they've yet to sign off on this plan because they fear the enforcement mechanism contains too many defense cuts.
This, of course, is the entire point of the enforcement mechanism -- to make it so onerous to all parties that they make sure not to pull the trigger on it. It's a key tell that Republicans are making their last stand here. They want to weaken the side of the enforcement mechanism that puts them and their interests in an uncomfortable spot. That way, they set things up so that if they oppose future deficit legislation (i.e. because it contains tax increases) they can walk away and take across the board cuts that they're comfortable with.
That's where we are at the moment.