There is indeed a long tradition that the RNC, post convention, will hand the reins to the nominee. This usually involves the movement of a portion of the campaign staff onto the RNC payroll, including some in senior, directing positions. But that is a matter of voluntary deference. The RNC is legally independent of the nominee and in theory could choose not to accommodate him. That would be unusual, but Trump is creating all sorts of unusual situations.
Shouldn’t there be a National Committeetranny today?
Right. so why antagonize the chair, if you have power over it anyway.
Maybe the RNC helps trump 70% and then provides the rest of the support to senate/house.
Then Trump just whines about that too when he loses in the fall.
Why should he change now?
Presidents have traditionally been given their choice for Chair of the RNC, but then, candidates traditionally haven't trashed the RNC itself. So I'd expect that the people in the party he's been trashing would like nothing more than to stick one in his eye and pick someone he doesn't like.