As has been said, you need a major, major POD (or even several) for Goldwater to defeat LBJ in 64. In addition to the points already posted, the JFK death effect was also a strong factor. Goldwater himself stated 3 Presidents in 12 months was hard to image.
BUT, since this is ALT History...
The only way, assuming no POD till the actually 64 campaign;
1. RFK splits the party, We know RFK never liled or trusted LBJ, someincident (either private or public) cause the final break in 64, vice 65 drives him to run against LBJ or at least hold back his support. (Very unlikely, this is still the age of party loyalty, RFK does this and he better win or start his own party afterwards).
2. LBJ's Real life story come out. NOT GOING TO HAPPEN, the media was in lock step with LBJ in 64 and even if they where, there still the ghost of JFK looming.