the thing is, that kind of unity that QAnon has brought (white supremacists, dudebro magats, and anti vaxxers) isn't something new. The seeds for those different movements all come from the same source: that is, anti intellectualism, faux populism, racial victimhood narratives, etc. that kind of irrationality thrives off of human biases, that is, it's essentially like a cult or religious movement. it's pretty hard to defeat that kind of irrationality when the person in question won't even begin to listen to a rational explanation. trust me, i've tried with my own family and people online. it eventually loops into circular arguments, moving the goalposts, etc. the right's entire thing is about oppressing people, or as libertarians would imply, "the right to oppress other people". even if the methods are different (e.g oppression through a totalitarian state, or oppression through capitalism), the end result is more or less the same.

it's not like the left isn't completely fractured, but the differences are definitely tangible. IMO the main thing that unites the left would be to resist capitalism, and to promote progressive values. other than that, i'm not sure.