A subgroup of people on the left who believe in communism and mostly hold pro-Russia and pro-China views, while often having a "doomer" mentality in regard to the US.
Unfortunately, that has them made very susceptible to Russian propaganda, to the point where they're now doing the bidding of Russia and helping fascists rise to power.
The mechanisms are similar to MAGA. They've disconnected from classical media and their echo chambers censor posts that highlight positive developments in the US or posts critical of Russia/China. Once inside, their world view collides with the outside and it's hard to get out again. Similarly to Russians and Republicans, they vilify liberals ("liberals are complicit in xy", "scratch a liberal and a fascist bleeds", "Marx warned liberalism inevitably leads to fascism", etc.).
On here, they're largely the people dissuading US Democratic Party voters from turning out, via "both sides bad" and recently via claim-to-purity (I'm sure you've encountered one of those "genocide joe" posts, which are kinda awkward, since tankies commonly support/deny China's genocide on the Uyghurs and Russia's genocide on Ukrainians).