Goldwater lost Ohio by the largest margin for a Republican in recent history, taking only 37.1% of the vote to Johnson's 62.9%. Obviously fundamentals would be more favorable to Goldwater in 1968, but I don't see how Goldwater could possibly keep it close in a state he lost by over 25%. Especially since Humphrey nearly beat Nixon in Ohio in 1968, losing by only 1.8%.
In 1964 Goldwater was running as the nominee of a badly divided party against an incumbent president at a time of peace and prosperity who also had the sympathy factor of having inherited the presidency after an assassination. The situation is radically different in 1968 where it is the Democrats who are badly divided, while the country is mired in an unpopular foreign war and is seeing dramatically rising crime and racial unrest at home.
Moreover, it's been suggested that Goldwater in '64 knew he didn't have a chance to win the election and thus felt free to talk freely about his beliefs. (Leading to things like him describing himself as an extremist in his campaign speech, talking about privatizing the TVA in Tennessee and abolishing farm subsidies in the farm states, etc.) A Goldwater that actually has a chance to win the election is likely to be (somewhat) more guarded in what he says and thus won't be damaged by as many gaffes as he was in his '64 campaign.
And of course Humphrey is much less likely to go for the jugular against Goldwater than LBJ did. (This is after all the man who wouldn't even publicize Nixon trying to sabotage the Paris Peace Accords IOTL, so I really don't see Humphrey allowing anything like the Daisy ad which did so much to paint Goldwater as a dangerous extremist.)
Thus when you combine all these factors Goldwater should run much, much stronger in Ohio in 1968 than he did in 1964.
And note that Wallace hurt Humphrey more than he did Nixon in the north.
What's your source for that? I've seen numbers for Wallace's impact in the southern states but never in the northern ones.
Even then, I seriously doubt that Goldwater would defeat Nixon at the 1968 convention.
I sort of agree with you here. For Goldwater to win the nomination in '68, you probably have to have Nixon end up as the Republican candidate in '64. (Which isn't necessarily that improbable. If Rockefeller wins the California primary in '64, it won't be enough to get him the nomination, but it might be enough to derail Goldwater's candidacy and lead to Nixon being drafted as a compromise candidate.) Once Nixon loses in '64 (his third major loss in 4 years), he is done. No one would take him seriously in '68.
If anything, a Goldwater candidacy makes Nixon's nomination more likely by splitting the conservative vote with Reagan.
Reagan may not even be a factor if Goldwater isn't the GOP nominee in '64. (No Goldwater presidential campaign that year means Reagan doesn't make his national speech that got him so much attention, and thus Reagan may not even run for Governor in '66. Odds are George Christopher is the Governor of California instead.)