Once upon a time, when I was a lonely teen rotting away in a rural high school, we spent a year grazing American history. One of the numerous events that were a mere side note on our speedy journey through time was the 17th Amendment to the US Constitution, which provided for direct election of senators. Prior to the amendment, senators were elected by their respective state legislatures.
I have been thinking about this quite a bit of late, while watching a few recent political struggles. It seems truly odd that a law passed by the federal government could overrule and weaken an already-existing law instituted in a particular state. Especially since the 10th amendment states, "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." This would seem, on its face, to provide two completely separate sets of powers, one for the federal government, and a different one for the states.
It seems to be the case that the current situation, where a federal law can overrule a state law, is a long-term result of the 17th Amendment. Prior to 1913, the fact that senators were elected by the states provided an implicit check-and-balance against the popularly elected House of Representatives. If one party could influence the common man, they could sweep the House, but they would find stiff opposition from the Senate for any plans to expand federal powers at the expense of the states. After the amendment passed, influencing the opinion of the common voter was sufficient to gain control of the entire Congress. It's not completely easy, because only a third of the Senate is elected at a time. But with the current division in American politics, this is typically still sufficient to switch control between parties at any given election.