I don’t know if this is universal, but all the people I know who consider themselves antifa in Connecticut also actually consider themselves far-left (anarchists or some variety of communists, mostly), myself included. I’d love it if liberals would take part, but I don’t know any self-described Democrats or moderates who do.
To be fair, the name Antifa is used pretty much exclusively by radical anti-authoritarian left groups.
The libs don't organise into black blocs to beat up fascists, they write to their congressman to ask for stricter hate speech laws.
The tankies, despite creating the first group to call itself AFA, don't participate in them much in the modern day because they try to subordinate such things to their party, which goes against the decentralized ethos of what most people call Antifa.
This leaves anarchists, libcoms, and a small smattering of demsocs and Trots. Thus it is completely fair to refer to "Antifa" as a left-wing phenomenon.