Anyway you look at it, it doesn't make sense to support a political party that allows non-party-members -- even Republicans! -- to choose its presidential candidate.
American workers -- especially those who are poorer and have less formal schooling than Iowa Democratic caucus-goers -- need a political party that represents their interest first and foremost, a party that really functions in a true partisan fashion, excluding non-party members from decision-making and constantly involving all party members in decision-making between elections.
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What your're calling for is what the rest of the world would consider a party, but what in America is considered a club or a pressure group. Thanks to a number of reforms in the past century, political "parties" in the US are public institutions that anyone can join, not membership organizations. (Public institutions that are dominated by private -- mostly corporate -- interests, but hardy unique in that respect.)
Whether or not working class organizations should be politically active in Democratic caucuses or primaries is a matter that should be decided by circumstances. But the Democrats shouldn't be ruled out just because they let anyone "join" them -- that's true of any "party" with ballot status.
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