Cliff has no reason to say no besides the final 4 agreement, but if Nicole reminds him that Holly and Jackson were playing the field on that agreement when Christie was still around, he should hop on board the plan.
Especially when this move can be made with little to no blood on his hands.
He and Nicole are partners in crime, but if she’s taking the shot, chances are Jackson will be the one out for revenge on her if he does somehow survive elimination yet again.