who proposed this amendment?
The 62nd Congress.
Seventeenth Amendment to the United States Constitution

The amendment was proposed by the 62nd Congress in 1912 
who elected senators under the provisions of the constitution as originally ratified UNANSWERABLE what is this aforementioned amendment? An amendment for the popular election of United States senators in each state. when was it established? 8 April 1913.
wiki:19623573
Seventeenth Amendment to the United States Constitution [SEP] Introduction
The transition began with two special elections in Georgia and Maryland, then in earnest with the November 1914 election; it was complete on March 4, 1919, when the senators chosen at the November 1918 election took office.