The massive and organized effort to steer people to the alt right, plus, the distortion of information used by them.
Good question.
I'm going to make some broad generalizations, so take what I write with a grain of salt.
I think there are some fundamental differences between left and right that ensures that the left can't use the same tactics as the right and expect to get the same results. In essence, the teams are playing by different sets of rules, and the left side is playing with a huge handicap. From a high level, the right is typically more focused on results than process, whereas the left views
process as the result. For instance, a right leaning person might be more comfortable with a morally corrupt politician ignoring rules in order to achieve a result whereas a left leaning person cares that rules are followed, because they believe
the rules are set up to produce the best result. Think of voting as an example: the left wants open, free elections, and the right is more comfortable with voter restriction laws, gerrymandering, etc., because they are more concerned with results than process. The left and right approaches to debate is another example.
You can apply this rationale to alt-right tactics. They might be more comfortable with manipulating algorithms or bad faith arguments. Something like crying "censorship" on a platform is a pretty effective tactic for the alt-right because they know it is more effective on convincing liberals to acquiesce mainstream spaces to spread ideas.
So back to the question: what can be done? Short answer: nothing. Alt-right tactics are not a thing to counteract, but rather a new paradigm that has to be adjusted to.
Long answer: I guess my hope is that the left becomes educated on the tactics and call them out. If you believe truth is on your side, you need to be able to defend it intelligently. You might have to get used to debating things that should have been resolved a long time ago, like scientific racism or even flat earth nonsense. And you have to get used to the reality that some people are beyond reason and decide whether or not its worth the time to try to appeal to their pathos. Your energy is better spent working with willing partners rather than try to persuade the obstinate.