Well, the first erosion of the unlimited debate in the Senate came with America's entry into the First World War. A group of anti-war Senators led the longest filibuster in Senate history, and the Senate rules were re-written mid-session to create the Cloture Rule allowing 6/10ths of the Senate to agree to end debate and proceed with the vote.
A different political climate at the time might lead to the Senate completely rejecting its privileges of unlimited debate, and imposing more structured, House style rules.
Yes, I'd forgotten about the importance of WWI in this whole process. Like eveybody else I associate the fillibuster with the preservation of Jim Crow.
MNPundit said:
Actually just have the argument made on the left side that the filibuster should be abolished (in totality) win the day when the GOP talked about doing it for judicial nominations.
Let's have the GOP senate conference adopt an anti-filibuster policy during one of the eras when they were riding high.
F'rinstance, if Democrats & insurgents successfully filibuster the treaty that came out of the Washington Naval conference (why? How? Let's handwave a strange alliance between ultra-isolationists, militaristic Dixiecrats and senators from shipbuilding states) then the GOP leadership might ask VP Coolidge to rule that Rule 22 be rewritten, or even abolished.
Otherwise there's my idea of the progressives sucessfully filibustering Taft-Hartley in '47, leading to payback when the GOP reclaims the majority and the senate president's chair in '52.
What is the overall effect?
I can't help but think that some of the US upper house's greatest debates that led to the rejection of controversial policy occurred without the invocation of the filibuster. Think the League of Nations, or the court packing controversy.
You don't need a filibuster to thwart executive overreach--but there are sections who define 'overreach' differently to you and me. The Southrons, obviously.