Wow. I thought it was mostly the South.
Mostly-- but consider this...by 1963 the North had allowed, for close to 100 years, the South to return Black Americans to a state of near slavery, run a one-party state, enforced through state terror and paramilitary violence. And the Civil Rights movement didn't lose steam until it ran up against the wall of
de facto economic, social, and geographic segregation in the North.
Was it as controversial in the North to support the Civil Rights movement as the South? Well, no, of course not-- but that mainly meant you weren't likely to get shot or beaten over it, not that racial equality had become popular or mainstream by 1963 in the North...it was getting there, but not quite there yet.
And for an actor like Heston, there were plenty of movie theatergoers down South too, dig?