Joseph/Haleivi: The idea is that, state legislatures will not like when the federal government tries to take power from the states. So if the state legislature is choosing the senate, senators will be careful not to make them mad.
I don’t know why anyone would be opposed to this, since if you want certain policies in your state, you can just legislate it there, and not care what the rest of the country is doing.
(for example socialized healthcare would have zero constitutional challenges if implemented on a state level. It wasn’t- because nobody really wants it. )