Most Catholics (other then Italians) were Democrats at that time, someone specifically who would have jumped from Smith to Hoover would have to be someone with a particular axe to grind like a Tammany Hall oriented NYC Democrat who was aligned against FDR's upstate wing of the party and maybe had been denied a patronage job.
What explains the drop-off in support among Catholics in Catholic Southern Louisiana? There was no third-party candidate to possibly explain why FDR got a smaller percentage of the vote in some counties in Acadiana than Al Smith did, in case that's what you were thinking. Several counties fell from >80% Smith '28 to >70% FDR '32.
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