The framers of the Constitution, meeting in Philadelphia on June 26, 1787, made two key decisions about Senate operations. They established the term of office and the source of compensation.
A majority of convention delegates shared James Madison's view that the Senate needed protection from momentary shifts of public opinion. The best protection would be a lengthy term of office for its members, without possibility of recall. Madison thought nine years would serve that purpose, with one-third of the seats expiring every three years. When put to a vote, however, his proposal lost by a large margin. With other delegates pushing for just four years to keep members from losing touch with their constituencies, the convention adopted a six-year compromise.
The convention then turned to the touchy issue of members' salaries. Benjamin Franklin believed the Senate should represent the nation's wealthy classes; therefore if no salary were provided, only wealthy persons would serve. He warned that if the convention authorized salaries, the public might suspect it of having "carved out places" for the younger delegates who would be natural senators. When put to a vote, Franklin's proposal nearly won, with five states in favor and six opposed.
Then the question arose as to who would pay the salaries. Several delegates suggested the individual states. Madison reminded them that this would destroy the principle that senators were to be "impartial umpires & Guardians of justice and General good," reflecting national as well as state interests. It would also subvert the plan for a six-year term, as states could in effect recall senators by withholding their salaries. This motion also failed by a five to six vote.
The framers subsequently decided that members should be paid out of the national treasury, but—in their wisdom—left it to the new Congress to decide how and how much.
Kurland, Philip B., and Ralph Lerner. The Founders' Constitution. Chicago: University of Chicago Press, 1987.