>Now, in a hyperpartisan environment, reelection could often be the motivating factor, with little incentive to cross sides.
Oh no, thanks to democratic elections of senators, senators risk their voting decisions being scrutinized by their constituents...yikes!
More democracy more directly is not a salve that cures all problems. Election of judges is pretty crazy, for instance.
IIRC, senators were originally appointed by state legislatures, and were understood to be representatives of the sovereign state governments themselves. Sort of like how the US government sends a representative to the UN rather than having the people elect one.
The problem is when representatives have more allegiance to their party than to their constituents. That's a long term side effect of plurality voting (which forces everyone into two opposing sides, lest they lose due to vote splitting). It's also very solvable, such as with Approval voting [1] that is not subject to vote splitting and tends to elect the first choice of the hypothetical median voter.
1. now in Fargo ND! :)
Senators appointed by state legislatures could be counted on to oppose any attempt by the federal government to harm the states. Today we have huge problems with unfunded mandates and with funding being used to coerce states into making state law.
Example: states don't get highway funds unless they make the drinking age at least 21
That kind of stuff is all over our law. With senators being chosen by the states, it wouldn't happen so much.
BTW, regarding the constitutionality: our constitution prohibits a state from losing representation except by consent, but not all states ratified the 17th Amendment.