If they are anything like our liberals, they will sit on their hands and do nothing; and then, when the Nazis win outright, scream very, very loud about how it's the Left's "extremism" that pulled the "centrists" rightward, and that the Left must be allowed to be purged completely so that a new "center" can emerge as the new "liberal" opposition.
Left simply needs to assume they have no allies, defend themselves, and organize the marginalized.