Couldn't I vote for Biden then for the same reasons?
I don't conceptually understand how you could frame a case for doing this. I've never seen a Traditional Catholic (And I'm being really broad here) even try. And usually when Novus Ordos do, its something like "sure Biden supports child murder and whatnot, but like, Trump isn't perfect on abortion either, but look at all the *other* Catholic things the Democratic Party agrees with like no death penalty and allowing more immigration and being pro "social justice" and BLM and... you get the idea. Stuff that isn't *even* really Catholic, just the more liberal side of the NO.
Honestly the only reason I could even fathom to vote for Joe Biden would be something purely realpolitik and having nothing to do with the candidates themselves... something like "if Biden wins that will divide our enemies into Neoliberal and Progressive factions" or something like that. I personally wouldn't do that, but I'm not sure I'd accuse someone who did decide to pursue that strategy of mortal sin.
But otherwise.... like what argument could you even construct for doing that? I suppose maybe if you sincerely thought Trump was going to bring about nuclear annihilation, that could trump the evil of a pro abortion position, maybe and arguably (not certainly by any stretch.) But the evidence is against that too, Trump has been comparatively restrained on foreign policy too.
To be clear, I'm not even trying to argue *for* a Trump vote, just wondering how we can make an accusation of grave sin.