The de facto supermajority threshold was first forged against civil rights. Jim Crow-era segregationist senators repurposed a 1917 Senate rule to force every civil rights bill to clear a supermajority threshold, blocking them all. Only civil rights bills were blocked in this way.

Yes! Calhoun was profoundly racist. He was slavery’s leading defender in the Senate. He argued on the Senate floor that slavery was a “positive good.” And he was motivated to innovate the filibuster by the desire to protect slavery - to give the South veto power. Bad, bad guy. https://twitter.com/mcbyrne/status/1352279151349293059

In Kill Switch I show how the Senate now applies the standard it used to block civil rights to all issues. There’s no wisdom in this delay, just plain old obstruction. That evolution runs though Richard Russell (a white supremacist) and Mitch McConnell. https://wwnorton.com/books/9781631497773

Yes! In Federalist 62-63, Madison lists all the ways the Senate was supposed to foster wisdom (partisanship per se didn’t exist then). None of them were the filibuster - not just because it didn’t exist, but because Madison believed majority rule was the “republican principle.” https://twitter.com/nbeaudrot/status/1352325779976192002