The American Revolution, the Founding Fathers, the Articles of Confederation, the Constitution, the establishment of the United States of America.... was the first, biggest, and most signficant and radical Leftist movement in history. I mean, heck, they cast out their God/King. They were subjects of the British Crown, its property. This was totally unimaginable at the time. This was a world that was just barely out of the Medieval Era, and in many corners, still very much was.
Sure, Russia, China, etc may seem rad, but basically all they did was get rid of one king and get the serfs another ruler, as explained in the historical document, Animal Farm.
Mind you, the Founding Fathers were about to do the same thing - crown George Washington - then went for establishing a new feudalism with them as the new aristocratic class, the new lords. They appointed the first presidents. They appointed delegates to Congress - well, state legislatures did, but those were made of aristocrats also.
When they later decided to grant the vote to citizens, it was only to WASP male monied landowners - and that only reluctantly, with the Electoral College established to 'rectify' the vote if need be because they thought even those men of means, commoners they were, still, were too stupid to vote wisely.
So there was an evolution, not this grand idea that people staged a revolution for as the mythology goes. And that development - progress - was spurred by Liberals, and it is still going on today, despite the continued efforts of Conservatives to stymie it since 1775.