What must be kept in mind is that a political party is a protected organization under the First Amendment's freedom of association. Thus, the party's rules for procedure can be developed and altered as the party itself best sees fit without interference from the state.
It is also interesting to see the movement to stop Trump as accelerating. I agree. Sadly, major media writers, who seem now to be little more than (mostly) men with a pen, cannot be trusted to describe the inner workings of a major political party. More here: What a Party Convention Nominating Process Really Is .