…one can vote for Biden/Harris not for the purpose of keeping abortion legal, but for the purpose of preventing an existential threat to the nation…
That is entirely a matter of opinion. I happen to think sincerely that the threat is real. There is nothing in Catholic teaching that says I am wrong. And I have seen no pragmatic argument that says that I am wrong either. Those that say there is no such threat are just expressing their personal opinion too.
Abortion is legal because it is constitutionally protected by the SCOTUS ruling on Roe.
And the reason the SCOTUS ruling still stands is that a majority of people support it. I do not. But a majority does. And if somehow a ruling is forced through that is not supported by a majority of people, it will likely suffer the same fate as prohibition, which was also forced through contrary to the will of the majority. On the other hand, if the population is first convinced that abortion is an unnecessary affront to human life, it will be a lot easier to get compliance with a law that makes it illegal.