If you are referring to the laws put in place in the late 19th and early 20th century, than the answer is the direct election of US Senators.
During the early 20th century, America was experiencing a significant political change which resulted in the Progressive era. This Progressive era was focused on giving individual citizens more political and social rights. This included helping women get the right to vote and ensuring that immigrants and blacks got to vote for their senators. This became a reality thanks to the passing of the 17th amendment of the US Constitution.
