Which most directly led to the end of hierarchical parties led by strong party bosses?

Question

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The Shankman Decrees were a set of regulations put forth by the Democratic Party that restricted the power of party bosses and led to the end of hierarchical parties led by strong party bosses. Party-centered campaigns and the Cursus Honorum (a system of political advancement in ancient Rome) do not directly relate to the decline of strong party bosses.

Which Most Directly Led to the End of Hierarchical Parties