Given that Nixon in 60 was basically Eisenhower, but more openly pro-Civil Rights?
George Wallace.
George Wallace isn’t getting the Democratic nomination in 1964, or any year. No Southern candidate* got a presidential nomination for a century after the Civil War, precisely because of racial/civil rights issues. Black people were just too important to the coalition, even then. Kefauver, a modestly pro-civil rights Southerner, couldn’t get nominated; Richard Russell, a staunchly anti-civil rights Southerner, couldn’t get nominated either. LBJ could only get the nomination after he was already president. And he was the second most pro-civil rights Southern senator of his time (behind only Ralph Yarborough). If Wallace was nominated, Black voters would abandon the party en masse, hurting the vote in Northern cities, and liberals would vote Nixon or simply not show up.
The Democrats of that era were well aware that having their nominee pegged as a “regional” or “sectional” candidate would be deadly, and they simply wouldn’t let it get to that. The party bosses would never allow it, for one thing.
It’s not like Goldwater. Goldwater could get nominated despite his extremist views and unpopular qualities because the conservative movement deliberately moved into the GOP in large numbers during the early ‘60s. But that just wouldn’t happen with the party of FDR, big government, and labor unions. The conservative Republicans disliked Rockefeller, but Walter Reuther might as well have been Stalin.
I won’t say it’s impossible — this is alternate history, anything is possible! — but your POD would have to either be further back than 1960, or something a lot bigger than Nixon winning, to get a President Wallace.
That’s my opinion at least.
*Woodrow Wilson was a Southerner by upbringing but by the time he entered politics he had long since left the South and come to represent New Jersey.