With a POD no earlier than 1820, have Libertarians (socially Liberal, economically minimalist) and Communitarians/Christian Socialists (socially conservative, economically leftist) become the two dominant political movements in the industralized world, with most people ascribing to one or the other and with OTL Left and Right as fringe movements (like Libertarianism and Communitarianism today).
Basically, instead of every country having a dominant left party and a dominant right party, every country has a dominant Libertarian and dominant Communitarian party.