An old post of mine at
https://www.alternatehistory.com/forum/threads/wi-humphrey-in-1960.413130/#post-14486878
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I really have a hard time seeing Humphrey, with his very liberal reputation--especially on civil rights--carrying TX (where the oil industry will be violently hostile), NC, SC, or LA or getting any electoral votes in AL. (As it was, the Democratic state committee in LA *almost* voted to support unpledged electors instead of JFK; they would almost certainly vote to support unpledged electors against HHH.) And I don't see him making it up elsewhere; the big industrial states were mostly carried by JFK anyway. The same is true of MN, and most of the other farm states were sure to go Republican. Finally, while he was anti-Communist (like most liberals in that era) probably no Democrat could be "tougher" on communism than Nixon. Or if any Democrat could, it would be JFK (who actually talked tougher on Cuba than Nixon, who of course could not talk about the planned invasion...).
It's hard for me to see him winning the Democratic nomination anyway. He lost the WV primary, even though the state was overwhelmingly Protestant and had fond memories of the New Deal (which Humphrey tried to evoke, even having FDR, Jr. campaign for him). And even the votes he got there were not necessarily really for him. Robert Byrd, an avowed LBJ supporter, said "If you are for Adlai Stevenson, Senator Stuart Symington, Senator Johnson, or John Doe, this primary may be your last chance to stop Kennedy..."
https://books.google.com/books?id=w3oiOriupLwC&pg=PT209
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To that I would only add that there might be considerable resentment from Catholic voters who thought the Democrats rejected JFK because of his religion.