It is true that liberals and leftists both want to make society more economically and socially egalitarian. But liberals still hold to the classic Enlightenment political tradition that cherishes individuals rights, freedom of expression, and the protection of a kind of free political marketplace. [...]
The Marxist left has always dismissed liberalism's commitment to protecting the rights of its political opponents [...] as hopelessly naïve. [...]
The modern far left has borrowed [this] Marxist critique of liberalism and substituted race and gender identities for economic ones. [...]
Liberals believe (or ought to believe) that social progress can continue while we maintain our traditional ideal of a free political marketplace where we can reason together as individuals. Political correctness challenges that bedrock liberal ideal. While politically less threatening than conservatism [...], the p.c. left is actually more philosophically threatening. It is an undemocratic creed.
It's becoming increasingly difficult to maintain safe, free discourse in a public square that frequently believes differing opinions (and the free expression thereof) are not ultimately the basis of democratic systems. This doesn't just apply to politics, either, as we see the increasing bifurcation of online communities (namely Twitter) over these issues of free speech, diversity, and respect.