I don't think Lake County, MN is that difficult to explain. The Mesabi Range attracted a large number of radical, largely Finnish workers, and for that reason the county was a Socialist stronghold. One major reason FDR didn't carry it in 1932 was the large (19.32%) vote for the Socialist candidate, Norman Thomas.
https://en.wikipedia.org/wiki/United_States_presidential_election_in_Minnesota,_1932 These were the sorts of voters who in other states voted for FDR. By 1936 the Socialist Party was in steep decline and organized labor was overwhelmingly pro-FDR, so it was natural that FDR would carry the county although the huge margin by which he did so suggests that he converted ex-Republicans as well as ex-Socialists. (One reason Lake County was so heavily Republican in 1920-28 and gave Hoover a plurality though not a majority in 1932 is, I would guess, that a lot of German-Americans resented the Democrats as the party that had brought the US into war with the
Vaterland.).