They would if they sincerely believed that losing this election would doom democracy. If they nominate a straight polarized political ticket instead of one that can actually unify everyone who doesn't want to see a second Trump term then we know that they don't actually believe their own apocalyptic rhetoric.
Nominating Harris with another left-leaning running mate may make the most sense from a purely strategic perspective, but I don't think that's because anyone seriously expects her to win at this point or because it's actually their best shot at protecting democracy from the impending apocalypse. They're just hoping that having her in the president slot leads to fewer losses in the rest of the races.
Of course, whoever they nominate will just be called a leftist anyways. Even Harris is on the right wing of the Democratic party, and here you are calling her "left-leaning."
[1] https://www.politico.com/news/2024/07/21/harris-potential-vp...
People keep repeating this, what the heck does it actually mean? If we define Kamala Harris as "left of center" what does it actually look like if she "moves to the right"?
Just listening to the news/social media for the past decade, every single "right wing" issue that gets brought up is purely performative culture warring, so what do these "centrists" want?
Seriously, if you look at her record, she's very center or even right of center. We're so polarized now though that the far right is straight up fascism.
Romney would be an interesting choice. I remember this[0] article in the globe where a columnist who dragged him during his presidential run apologized after seeing how bad things could be.
Like you say though, hard to imagine any party conjuring the courage to do something like that even if they thought it was wise.
[0] https://www.bostonglobe.com/metro/2016/07/20/sorry-mitt-romn...