Congress passes bills that they want states to follow. Sometimes there is a direct dictate, sometimes it has to be with something of a carrot/stick approach.
Example: Back in the 1980s all the states had varying drinking age laws. Some 21, some 18, and some in between. Congress wanted all states to have 21 as the minimum drinking age. Problem was, Constitutionally they couldn't do it. It would have been struck down and they knew it. So, they used highway money as their carrot. "If you don't raise your drinking age to 21, we'll not give you any highway money." Not all states wanted to comply, but states like their highway money, so all the states complied. Blackmail.
That's just one example. There have been others. And it's not always highway money, but that does seem to be a favorite. This works for unfunded mandates, as well, Congress telling states they have to do something then leaving it up to the state to figure out funding.
Congress, which is the House and Senate combined, does this. Congress is popularly elected. There's no fear of retribution because (theoretically) the people's ox isn't the one being gored. If the Senate were still state appointed, then the Senate could stand up and say, "No!" The Senate would be representing the state's interest, and the people would still have the House. A balance of power. The way it is now, there is no balance of power, so of course the interest with no representation is going to be on the losing end.
