Democratic lawmakers have faced eruptions of anger at town hall meetings across the country this week, as constituents have coupled their fury over President Donald Trump’s actions with deep frustration over what they see as a feckless Democratic response.
Presidential primaries are the weird ones because they’re spread over all the states. Otherwise candidates just need to do whatever to get on the ballot(usually gather signatures) and then win the election. Parties can throw their weight behind one candidate in particular but that’s not the same as declaring a winner. AOC got into Congress by beating a party backed incumbent in a primary.