I hesitate to necessarily pigeonhole the parties pre-1896, because the concepts of left/right become foggier viewed through today’s ideological lens. Each of the parties had their differing wings and factions from the beginning, some held longer than others and evolved (Jeffersonian Republicans became Jacksonian Democrats) or collapsed (Federalists, the irreconcilable split between the Cotton and Conscience Whig factions and the pro- and anti-slave factions of the Democrats).
A fair and accurate assessment of party development.
My core point is direct and simple. Post-Civil War Industrial Capitalism evolved from the pre-Civil War Merchant Class. It was aided and abetted by a progressive mentality which the Republicans provided through their executive action, legislation and judicial rulings which centralized power in DC and eviscerated the vision of our Founders.