Republicans were cooler back when Marx liked them

Partly, but not entirely - there were two previous ideological shifts in the Republican Party - ~1870 -~1890, and ~1910 - which also detached the radical labor element from the Republican Party, making the control of capital over the party more total, and increased the core policy hostility towards workers - necessitating appealing to other prejudices to win over the working class votes necessary to win elections.