This is somehow connected to the fact that US political parties are mostly not mass membership organisations. Everywhere else in the world, if you strongly sup

This is somehow connected to the fact that US political parties are mostly not mass membership organisations. Everywhere else in the world, if you strongly support a party, you join it, and pay a membership fee, and the party adds you to a register. Then, you can take part in party activities, like internal elections. In the US, you register as a supporter of a particular party with the state. Then, everyone registered can take part in (some) party activities, like primaries. It's a weird system, and i don't know how it started, but because Americans don't realise it's weird, there isn't much written about it!