I agree that it makes sense in the abstract that ostensibly private organizations conduct themselves as they see fit. But in the U.S. the practical reality is t

I agree that it makes sense in the abstract that ostensibly private organizations conduct themselves as they see fit. But in the U.S. the practical reality is that the Democratic and Republican Parties are effectively official institutions. If voters' only input was to choose between two people secretly selected by these organizations, it wouldn't be very democratic. And imagine that happening in the many U.S. voting districts where the same party always wins - it would be completely undemocratic. On the other hand, IIRC my history, before maybe the 1970s the U.S. parties did use more private processes, selecting candidates in the 'smoke-filled room' (the image is of fat old men smoking cigars and making deals). Some say the quality of candidate was better then.