I’m not aware of a serious Left argument devoted to the idea that social democratic policy would easily win over Trump’s core supporters (i.e., the 30–35% of the current voting population that identifies as partisan Republican and has never wavered from Trump.) So far as I can tell, the premise has always been that these reforms would prove popular with the rest of the country. Combined with organizing drives and serious voting reform (AVR, etc), the promise of “universal free stuff” might help mobilize vast swathes of the apathetic non-voting working class (a larger and more left-sympathetic group than Trump partisans, many of whom are questionably “working class” in any case). This could, in theory, provide a sustainable majority in favor of social democratic reforms. Lots and lots of polls, as I’ve argued, seem to show about 60% national support for a Sanders-like economic agenda.
At any rate it might be more effective than Democratic approaches of the recent past — for all the tangible good that Medicaid expansion has done, it’s hard to argue that on a political level it has ever been pitched as universal “free stuff.” Joined at the hip to the very unpopular reforms necessary to protect the private insurance industry (like the individual mandate), the Medicaid bump was always folded into the complex and extremely un-free structure of the larger ACA. Nor, on a more fundamental level, is a subsidy directed at the very bottom of the social pyramid ever going to have the political power of a truly “universal” program.
Real universals are things like Medicare-for-all, free public college, or, say, an extremely popular Full Employment Act. (I’m with the anti-work Left in the long run, but in the meantime, anyone who wants to work should have a job.) This stuff isn’t going to win over hardcore Trumpists, but I think it could drag in the rest of the country, and build a foundation that would significantly diminish the power of Trump-ism in the long term.
Or maybe not! But I think that’s closer to the social-democratic argument, at any rate.