Goldwater's civil rights stance defines the modern Republican party. The party has moderated since Goldwater, of course, but Goldwater's positioning literally restructured US politics, from a de facto 4-party state (in the 1970s, Republican support for abortion rights was still an open question! There's a 1970s-era GOP platform where debate on abortion is a plank!) to the 2-party "liberal/conservative" one we have today. Rockefeller wouldn't even be a Kiwanis Republican today; he'd simply be a Democrat. Phil Gramm did become a Republican; in fact, almost every party switcher in the US after the Civil Rights act switched to the GOP, because they were fleeing the Democratic party after the Civil Rights Act.
The idea that the modern Democratic party is somehow accountable to votes against civil rights legislation is D'Souza-ism. It ignores the plain fact that those Democrats became Republicans, and, to a lesser extent, a bunch of liberal Republicans became Democrats. You see it clearly in the candidates themselves, but also even more clearly in shifts among voters.
Goldwater lost in a landslide, losing virtually every traditional Republican state including his home state of Arizona. He won a few southern states, but Carter won all of those same states in 1976. Saying that Goldwater’s civil rights stance defines the modern Republican Party is like trying to call Joe Biden socialist. It’s red meat for Democrats, not historically accurate.
The Reagan re-sort was driven by economic development in the southern states, as those states became dependent on low taxes and low regulation to draw business from blue states. Carter won rural Georgia in 1980, but Reagan won up and coming Atlanta suburbs like Gwinnett and Cobb.
On race, Reagan ran on opposition to affirmative action, which is completely different than the civil rights laws Goldwater opposed: https://www.washingtonpost.com/news/made-by-history/wp/2017/....