This is something that was defined in the Constitution, however. Article 1, Section 3 called for the selection of Senators by state legislatures. This is superseded by the 17th Amendment, and calls for Senators to be elected by the people of their states.
This is important to understand, because the 17th Amendment is an on-again-off-again political issue; Republicans have, in recent history, held most state legislatures, so repealing the 17th Amendment would basically guarantee that the Republican Party would control at least one house of Congress for the foreseeable future, and give the party greater control over who is selected to the office.
This is important to understand, because the 17th Amendment is an on-again-off-again political issue; Republicans have, in recent history, held most state legislatures, so repealing the 17th Amendment would basically guarantee that the Republican Party would control at least one house of Congress for the foreseeable future, and give the party greater control over who is selected to the office.