The Bill of Rights covers issues at the federal level that the states are not allowed to touch. However, you've forgotten the most important one. The Tenth Amendment. It applied before 1860 and although liberals and conservatives of all sorts have run roughshod over it to further their agenda, however limited it has become it applies today and in this case particularly
The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.As the election of Senators IS delegated by the Constitution, it does not fall under the purview of the Tenth Amendment.