Mamdani got some repub vote. The (class) issues demsoc (DSA) runs on are broadly popular beyond the Democrat voters. There’s a lot of demand fot medicare for all from republican voters for example. As more of them get disillusioned with the cult, some are likely to vote for the DSA rep running in an election near them. I don’t think republicans would turn out for DSA candidates en-masse but with the razor-thin margins some elections are won by, it could matter.