Leftism is unpopular by definition

This really depends how you define "leftism".

If you mean 'whichever side of politics is left of the population's center' then sure, it can't be a majority.

If you mean 'whichever side of politics is left of the political center' then that doesn't imply it's unpopular, and there's direct electoral evidence of 'left' parties achieving a majority government.

If you mean socialism and communism, they certainly aren't unpopular by definition. If anything, their definition makes them a mass movement of the proletariat, the vast majority of a post-industrial society.