@Olivier5,
I'm not quite sure why you think this differentiation is important or even real. As Trump now is the head of his party, Obama was earlier. And every president. A candidate, when successful, ends up there. It's part of the package.
But more importantly, the president needs to be there because he/she cannot go it alone and be successful in the position. He/she can't operate nationally without the party's support and cooperation. A candidate isn't running for King but for leadership of a broad coalition of citizens and existing operations at the state and community levels. It's not a "me" position but an "us" position. Even the maniacal egocentric Trump counts on others.