Fuck me, I ended up posting a wall of text.
wall of text
Before we start talking about alliances with the "Liberals" against the "Fascists", we first need to establish, what is a Liberal to begin with? Because this very simple question seems to beguile the vast majority of US political opinion-havers. The very first thing everyone needs to do is throw out the colloquial definition of Liberalism used in the United States. Liberalism has a specific meaning in political science, and hundreds of years of history to reflect on, going back to the "enlightenment." We have the writings of Locke, Montesquieu, Mirabeau, as well as the American "Founding Fathers," in addition to a couple more left leaning ones like Rousseau and Paine.
Specifically, Liberalism is the ideology of "representative" democracy, constitutionalism, "rule of law," private property and free markets. Furthermore, Liberalism is an idealist philosophy (in contrast with materialist philosophies such as Marxism). Idealist philosophy takes an approach starting from ideas, and believes the world can be shaped by ideas, whereas materialist philosophy takes the opposite approach, that ideas are simply a reflection of material reality.
Liberalism encompasses everybody from Bernie Sanders to Mitch McConnell. They are all Liberals. They all fundamentally believe in the constitutional institutions, "representative" democracy, free markets, "rule of law," and private property. They have significantly different opinions within this framework (i.e. how "representative" democracy should be, or how inalienable property rights should be), but none of them are advocating for the overthrow of the system. At the end of they day, they can be summed up as institutionalists. Bernie Sanders is just as much of an institutionalist as John Roberts.
Liberalism is a fundamentally bourgeois ideology. When push comes to shove, the liberals will always take the side of preserving the institutions as well as the status quo of social and property relations they represent. The more the status quo becomes untenable, the more Liberals will support violent, counterrevolutionary means to preserve it. In this way, the entire Liberal Democratic state slides inevitably towards fascism as Capitalism fails to serve the needs of the public.
This isn't something you should take personally. If you truly believe in social justice, if you know where you'd end up standing at the decisive moment, and you know it isn't with the fucking Fascists, you are probably not a Liberal. You have simply been inculcated in a world where Liberalism is the status quo, where Liberalism is portrayed as the bastion against evil. Where Liberalism is portrayed as "the long arc bending towards justice."
There are a lot of people who self-identify as Liberals who actually posses a much more radical potential, but Liberalism itself exhausted all of its radical potential with the overthrow of Feudalism. From that point forward, for centuries, it has operated as the justification of the status quo.