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.

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.

For me it is a frustration over that liberals would rather endanger the liberal democracy by working with the far right than collaborating with anything considered left. It is very obvious in Sweden. The swedish far right has declared that the liberal democratic project is a threat to their nationalist vision -- it's not just me as a leftist saying this, but liberals, as in the Liberal party, said this about them. Then came the last election, the Liberals sided with the far right. Its down to two liberal MPs and they could force a switch from the far right to just center lib politics. But no. They rather want prisons for children than work with a socdem and maybe suffer to have a cap on profits on charter schools.

As an outsider, I’m curious why there is such a focus on liberalism in leftist circles?

Liberal politicians in the US tend to be deeply connected with the industrial and media elites. So we get a recurring cast of candidates who are milquetoast on a slew of popular issues, while they're lauded as "The Most Leftist Politician To Ever Think About Running For Office" in headline after headline.

Leftists who run are regularly denigrated as unrealistic, unelectable, and disastrous for the domestic economy by the same industrial tycoons and media magnets who push unpopular candidates and their dismal policies. So we're in this constant state of tension during election season, with a candidate like Joe Biden who receives enormous stacks of cash and tons of DNC support fighting against an outsider like Sanders or AOC who divert time from expressing generally popular sentiments to argue over whether they're well-dressed enough to win over a rust belt used car salesman.

What’s the relationship between liberalism and leftism?

Liberals tend to campaign as leftists and govern as conservatives. So they initially attract a lot of leftist voters, and then end up having to argue that said voters shouldn't ask for anything from the party once the election is over. Leftists tend to live on the political outskirts, looking for anyone remotely attractive to rally around, only to get taken for a ride by con-men in the liberal party until they finally burn out and stop engaging with electoral politics.

But they have absolutely no desire to compromise with fascism, and they have the same criticisms of capitalism and the current market as myself.

This is pretty much the default stance for most people, I believe. The issue, is that without deprogramming the Anticommunist Red Scare Propaganda, and without reading Leftist Theory, this is the endpoint of this position, essentially doomerism.

That said, there’s still a shitload of liberals in the US that think we can simply vote these problems away and basically do nothing else. They aren’t willing to get their hands dirty if it comes down to it and will instead do whatever they can to fly under the radar and put on blinders. They fail to realize that the Democratic Party is the other side of the same fascist coin.

This is why it's important for Leftists to constantly agitate, organize, and spread theory. Electoralism is a doomed game, organizing is what's important.

Yes, that's why liberals have historically worked with fascists against Leftists.

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.