1) Goldwater’s landslide loss in 1964, winning no Republican states other than his own, proved that opposing the civil rights act of 1964 wasn’t tenable for either party. It’s not like Democrats could have beat Reagan had they opposed it. They would have lost their northern base, just as Goldwater lost the Republican base in 1964.
It’s one factor in the detachment of the south from the Democratic Party, but that detachment had been a long time coming due to economic changes.
2) By 1980, the debate shifted to affirmative action and quotas, where Republicans had a longstanding ideological basis for opposing government intervention.