I think that had Goldwater in '64 come down on the side of the Civil Rights Bill, as he was no raving racist but someone who just had issues with the scope of the bill, he probably loses the South.
It is possible that you get a third party Dixiecrat again to try to take some of the South. They stayed out of it this time, mostly because they as a faction were somewhat cowed and indecisive, and the disgust of many in the South to have to vote for a Republican was enough that you saw an unpledged Democratic elector movement come about in Alabama.
Had Thurmond, or better yet, Harry Byrd, tossed their hat into the ring, I don't think Goldwater wins any of the states that he did in the Solid South.
That leaves him only Arizona, and there was a substantial Democratic base there that was quite Conservative, which is why they backed him, but with an effort by LBJ to campaign there more, you never know.
1964 was probably the best chance in my view.
Nixon would not have won DC, no matter what.