Republicans have always been economic liberals even during Lincoln. Northern Democrats developed a heavy base of labor activists (unions), so started tending a bit left. Southern Democrats simply existed as “anything but Republicans” (the party of a much hated Lincoln), so catered to southern white supremicists.

Johnson’s push of civil rights in the 60s simply destroyed the southern democrat wing of the party, who were welcomed into the Republican Party via Nixon’s southern strategy, and the pro-union Northern Democrats became the sole power source of the remaining Democrats.

Now with Trump and his rust belt base, Republicans are losing a bit of their economic liberalism (being more against free trade).