Final answer:
The direct election of U.S. senators weakened the power of state and local party machines and their leaders.
Step-by-step explanation:
The reform that weakened the power of state and local party machines and their party bosses was the direct election of U.S. senators. Prior to the Seventeenth Amendment, senators were chosen by state legislatures. However, with the direct election of senators, the influence of party bosses over the selection of senators was diminished, as voters directly elected their representatives.