I will always point to mlk as a response to this question:

I have almost reached the regrettable conclusion that the Negro's great stumbling block in the stride toward freedom is not the White Citizens Councillor or the Ku Klux Klanner but the white moderate who is more devoted to order than to justice; who prefers a negative peace which is the absence of tension to a positive peace which is the presence of justice; who constantly says, "I agree with you in the goal you seek, but I can't agree with your methods of direct action"; who paternalistically feels that he can set the timetable for another man's freedom; who lives by the myth of time; and who constantly advises the Negro to wait until a "more convenient season." Shallow understanding from people of good will is more frustrating than absolute misunderstanding from people of ill will. Lukewarm acceptance is much more bewildering than outright rejection.

Because the last 400 years of human history basically has been liberals stabbing leftists in the back.

I needed an ELI5 really.

Liberals essentially cover for both capitalists and fascists. See, capitalist and fascist ideology are very unpopular on their own, so liberals come up with all kinds of ways to pretend that rich people owning everything is good for everyone (capitalism) or pretending that more police repression means more safety (fascism).

Liberalsm essentially acts as the pretend-friendly "facade" ideology of this unholy trio - so yes, it's simply coherent for leftists to despise liberalism.

To quote Malcolm X:

The white liberal is the most dangerous thing in the entire wester hemisphere. He is the most deceitful, he's like a fox. And a fox is always more dangerous in the forest than the wolf. You can see the wolf coming, you know what he is up to. But the fox will fool you. He comes at you with his mouth shaped in such a way, that even though you see his teeth, you think he is smiling.

All their supposed progress and opposition to capital only reinforces and propels capitalism, alleviating the need for fascism just for a little longer (which arises for the ruling classes when the majority of the population grows disillusioned with their lies, be they conservative or "progressive"). In the end only legitimizing the underlying framework (capitalism), without ever threatening it.

tl;dr: scratch a liberal and a fascist bleeds

Liberalism is a big-tent ideology that services Capitalism. Leftists want Socialism, Liberals want Capitalism. This is the divide.

Are you really trying to meme in a 3rd party candidate push? Really?

you must be fun at parties

And like. I’d consider myself a far left liberal, in the sense of how the U.S. defined liberal when I learned the terms, where it was more a place on the political spectrum, rather than a codified set of ideas.

The funny thing is, the US defined liberal is the codified set of ideas, liberalism is just to the left of the median in America. America is that far-right.

Right to left, I’d define the that spectrum as Reactionary (Alt-right), conservative, centrist, liberal, and revolutionary (leftist, I think?). I know that those terms have different meanings in other countries.

The problem is that you jump straight from Liberal to Revolutionary, there's a spectrum of thought among leftists. Revolution may be correct, but there are schools of reformist thought as well. Additionally, liberals and all those to the right of them are Reactionary, just in varying degrees. A "centrist" would be left of liberalism, ie a Social Democrat or Market Socialist.

I’d consider the Republican Party to currently be between conservative and alt-right, with the Democratic Party being centrist with liberal window dressing.

The Democrats are Neoliberal, there's no set dressing. Liberalism is just right-wing. Conservatives are far-right populists, ie fascists in some cases.

I think the U.S. political system is fucked. It was never intended to accommodate political parties, let alone the nearly 250 years of maneuvering by capitalists to slip reigns onto the government, which now appears to have fully succeeded.

On the contrary, the US was designed by wealthy Capitalists to benefit themselves. The system is working as intended, protecting Capitalists.

I believe that the embrace of fascism by the Republican Party is a means to control the ~60% of people who are left of center and without cohesive political representation because of limitations of the U.S. political system/bastardization of it/the pernicious influence of capitalism.

Fascism is a class-colaborative alliance between the bourgeoisie and petite bourgeoisie against the proletariat and lumpenproletariat along nationalist lines to attempt to forcibly return to a less-decayed state of Capitalism.

I don’t support the Democratic Party, nor do I really feel the U.S. government is in a place to fix itself without some foundational things changing. I don’t think, realistically, that those things can be changed without mass engagement and effort, which… sigh. I’m doing what I can.

Correct. Join an org!

But also, I don’t believe a revolt or some form of dramatic U.S. government reformation is possible. As a result, the folks that are already demanding change and have given up hope for reforming the system are hostile to me, and the other folks fall into the camp of being disengaged/only mildly upset or even desirous of a slide into fascism. It feels like there isn’t really enough people who are unified who want to change course without throwing the whole thing out.
I honestly feel kind of alone.

Reform cannot work, Revolution is the only way. Build up dual power, organize, and try to build up parallel structures. Organize!

What happens if the U.S. does elect Trump and it swings full fascism?

Beating Trump won't stop the conditions for fascism, only Leftism can. Fascism can only be kicked down the road, until the ratchet effect takes us there anyways, unless Leftists organize.

I’m not trying to challenge or debate anyone here. I don’t think you’re stupid, nor do I think the ideals are bad. I fucking wish society was more altruistic and smarter.
I just… don’t see any realistic or actionable outcome other than to keep fighting for every inch using the tools we have, even if they are faulty, entrenched systems.
Call me propagandized, unimaginative, cynical or stupid, or… whatever, I guess. I just don’t see other viable options, and I think broadcasting moral superiority, embracing divisiveness and exhibiting hostility is going to create roadblocks, should we need to unite. If we can.

You've got the core of it, but not the theory. Try reading Leftist theory! Whether it be Marxist or Anarchist, leftists have been attempting to fix the system and are growing in power.

How can the world become more altruistic and smarter when the hard right is actively destroying our public education system and right wing media causes stochastic terrorism with zero accountability?

Humanity isn't getting better. We are seeing the final results of secularization, the end goal of a godless world where all that was once sacred are now open season for mockery and destruction. And even worse is that so much of the left is actively to blame for this.

None of you will get it, you will just blanket downvote because you cannot grasp that religion has a necessary place in human culture and the social chaos we have now is partially caused by the mockery of an institution that has literally held together human society for more than two and a half thousand years of human history.

Apples and oranges. You can have fascism with any political system. The overuse of tankies is a indicator that the accusing party doesn't really understand that.

Leftists are building up political parties like the Party for Socialism and Liberation, volunteering for Mutual Aid networks like Food Not Bombs, or resisting ongoing genocide, like the Popular Front for the Liberation of Palestine.

Liberals just have to sit on their thumbs and support the status quo, Leftists have to do the hard work of actually building up dual power and organizing. These people also are allowed to meme on the internet.

Exterminating Gazans makes things better?

Maybe read the meme again.

When has voting kept fascists from power?

No. Leftism is primarily defined by support for a socialist economy. There is not a single liberal on the planet that would support socialism.

Liberals are not leftists if we define the status quo as capitalism and leftism as the progressive opposition to the status quo
(and those are the definitions I and probably any honest socialist uphold)

Nope, Liberals seek to maintain Capitalism, Leftists seek to move beyond it.

They are, or rather were. For most of the world, especially in Europe, liberalism means/meant socially liberal, i.e. left wing - based on personal freedom from imposition of others’ values on their personal and social lives. However, in America liberal has (relatively recently, as in 2000’s) become synonymous with neoliberal ideology, which is absolutely not left wing in any traditional sense, focusing on ‘small government’ and freedom of the markets—I guess because pronouncing two extra syllables is too much effort? Idk.

With the internet this peculiar usage has recently (as in the last 5-10 years) started leaking out of America and is being used in this confusing and ambiguous manner.

To be fair though, the Overton window has shifted so far right now that liberal (i.e. left of the nominal centre) shares much of the same space as neoliberal. See New Labour, and the current Labour government.

Edit: Deleted a paragraph that in retrospect was unnecessarily negative.