It started with the Biden Express TL.
Naw, I'm taking dibs on credit. I started the "Anyone but..." thing a year or so ago and did one for Lodge in '60 like a day before a trend burst. I plant my Norton flag in it.
Washington Senator Henry "Scoop" Jackson would have been a very good President.
Scoop Jackson has the problematic feature, which is frequently brought up in alternate history discussions (which is how I learned of it) that he was dull and would not have been a good campaigner. Good on paper, bad in practice in the age of the television president. On the one hand, he is going up against Gerald Ford who is not all that interesting himself. On the other, he'd be lack luster and have a chance of losing for it. And you also have the fact that he'd be running against Ronald Reagan in 1980 if he won: both were Cold Warriors on defense (Jackson has been frequently claimed as a proto-neoconservative), but Reagan would decimate him with charisma.
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Personally, I like the idea of Hubert Humphrey for 1976. It also has a Nixon vibe to it of the candidate losing and going into the wilderness for an administration before bursting back onto the scene, with a healthy dose of "we see what happened when we elected the other guy. we should have voted for you". The major problem for Humphrey would be his health, because he has bladder cancer which is killing him and which he needs to treat. It'd be a bit ironic because he first noticed it during the 1968 election but ignored it so he could run for that election. So that would be a potential hinderence to campaigning, and would certainly be a problem during his presidency were he to win in 1976. It could potentially force him to resign due to his health issues (I think unlikely since he would accept the nomination as it was even knowing of his situation) or he'd die in office or at least have a likelihood of doing so. Disease is always very fickle due to butterflies.