Why did the progressive movement support the direct election of senators?

The Constitution, as it was adopted in 1788, made the Senate an assembly where the states would have equal representation. Each state legislature would elect 2 senators to a 6-year term. Late in the 19th century some state legislatures deadlocked over the election of a senator when different parties controlled different houses, and Senate vacancies could last months or years. In other cases, special interests or political machines gained control over the state legislature. Progressive reformers dismissed individuals elected by such legislatures as puppets and the Senate as a "millionaire’s club" serving powerful private interests.