The incumbent party still holds a primary, even though most don't participate because the incumbent president is usually unchallenged, or is the obvious nominee regardless. I don't recall anyone polling as high as RFK Jr in an incumbent primary, at least in recent history. If Biden was a popular president with an actual base, someone polling 15-20% wouldn't even be a threat, regardless of low voter participation. However, who would actually come out to vote for Biden in a primary deemed "unimportant". If RFK supporters showed up and voted, he would be guaranteed the nomination. Am I missing something here?? Why isn't this talked about?