It's not that liberal democracies always slide, specifically, it's that Capitalist states always slide, and this is heightened by being in the Global North. Global North countries brutally explioit Global South countries via Imperialism, by relying on vastly under-paid labor and selling it in the Global North for higher prices.

Fascism is Capitalism in decay, the violent immune system employed by the Capitalist class. A great work on fascism is Blackshirts and Reds. I can provide a longer Marxism intro reading list if you'd like, but Blackshirts is a great start.

I also recommend Imperialism, the Highest Stage of Capitalism, and the famous Yellow Parenti Speech (a small excerpt here.

Primarily referring to Germany and Italy's descent into fascism, and we're currently seeing this happen in France, and now in the US. These countries only see a shift to the left with an external force, like Scandinavian states giving concessions to the working class when the nearby USSR posed the threat of a good example — and by extension, the threat of a working class revolution; of course, these concessions are gradually being taken away now.

Imperial core countries refers to colonizer countries that now control financial institutions like the IMF and World Bank, and depend on the continued exploitation of former colonies.

• How Capitalism Robs The Developing World
  ⠀

I specify liberal democracies in imperial core countries because we have seen limited successes for the left outside it. Like Allende coming to power in Chile (before being overthrown in a US-backed coup 2 years later), or now Lula and Claudia coming to power in Brazil and Mexico.

• Read Blackshirts And Reds

• Why The US Is not A Democracy

• Why Social Democracy Isn't Good Enough

• “Yeah Capitalism Is Bad… But What Can We Do?”

It's an interesting ending to an otherwise fine comment. Bernie would slide the US towards liberal democracy, further from fascism