The bicameral legislature was designed originally to be a lower house (House of Representatives) and an upper house (Senate). The lower house was beholden to the people and the upper beholden to the states. When the state legislature selects their representative to the Senate, that Senator is not beholden to anyone but the state legislature. The Senator was supposed to be above being a simple whore for a vote like the Representative was and to have a more Federalist system. State legislatures could recall a Senator if the Senator defied their wishes or voted against the interest of the state. We can do this with recall referendums, but that is a much more difficult process.
It was a way to help ensure that the states were more powerful than the Federal government. The senators were originally intended to be thought of as 'ambassadors from the state' and the varying make ups of the two houses of congress were to serve as yet another check on the general or Federal government.
Since the 17th amendment, the two chambers have slowly merged into very similar entities. The founders wanted very different makeups of the chambers for a reason.
In addition to Madison’s convention notes,
The Federalist Papers, another compilation of the thoughts of the Founders, can be used as a source from which to discern original intent. In
Federalist No. 39, Madison explained the combination of both national and federal characteristics contained in the Constitution. One of the national features of the Constitution was the provision that the House be elected directly by the people. He explained:
The House of Representatives will derive its powers from the people of America; and the people will be represented in the same proportion and on the same principle as they are in the legislature of a particular State. So far the government is national, not federal.
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The provision of Senate election by state legislatures. however, was one of the facets of federal character:
The Senate, on the other hand, will derive its powers from the States as political and coequal societies; and these will be represented on the principle of equality in the Senate, as they now are in the existing Congress. So far the government is federal, not national.
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