Truman 1948 MA

[Alcibiades]

The reason that MA was so close in 1932, despite having been one of only two Northern states to vote Dem in 1928, was the extreme polarisation between Catholics and WASPs. It was a very inelastic state, somewhat like Mississippi today, with WASPs being analogous to whites and Catholics to blacks. By 1948, this divide, while still undoubtedly still in play, had softened considerably, as Catholics became integrated into the white mainstream, allowing Truman to win the state more comfortably in a D +4 year.

[Alcibiades]

It might just be as simple as the demographics caught up.  Replace "Catholics" or "Irish" with "Hispanic" or something today, in a state like Arizona, along with some continued White defection from the GOP.  I remember seeing a quote that during the 1930s, alongside the fact that the GOP was being routed, Democrats were saying that the GOP coalition was too ethnically restrictive to survive in the future.  Turns out they were right, as the GOP made HUGE inroads with Catholics of both Irish and Italian descent over the next several decades.

This makes sense. According to the US Religion Census, in 1936 Catholics made up 39.9% of MA’s population, but by 1952 this had risen to 48.4%.