[amalcon]

This argument has always seemed odd to me. The 17th amendment first took effect in 1915, and the whole Senate had been directly elected by 1919. Are we really going to claim that the late 1910's are when things really went off the rails? For example, to your specific point here, it seems unlikely that this made amending the Constitution harder. Excluding the Bill of Rights and the 27th Amendment (each a special case for its own reason), there have actually been more amendments in the fewer years after the 17th was enacted.

It's also unclear why one would expect that the Senate would be less partisan if its membership were selected by state legislatures. State legislators have a lot more partisan loyalty than the rest of us, both because they tend to be more ideological and because they are deeply dependent on the party for future career prospects.