Your theory on why almost every presidential election was an electoral college landslide during that time?
well from 1920-28 it was because the Dems were on the side of WW1, the League of Nations and reigned over the depression of 1919-22.
From 1932-44, it was The Great Depression and WW2.
From 1952-88, it was because there was a middle that was 20% or more of the electorate. When one party nominated a bad candidate, that candidate lost badly. Today the middle is 5-10%. Today Goldwater, McGovern and Mondale would do no worse than 45-46%