I think we've have seen, at best, a number of other incidents of political violence - at worst, I shudder to think. But I don't think our first concern would've been the question of who the candidates of both parties would've been.

Though as for that: if, as Greg suggested, the Republicans had settled on a more or less reasonable replacement candidate such as Haley, then indeed Biden might not have felt such pressure to withdraw. And whether he had or not, I agree Haley could likely have won easily in November. But I don't really see how they would've picked Haley in the first place. Trump's fervent supporters dislike her even as it is and they'd be even less inclined to accept her or a similar candidate if they were further radicalized by their hero having been murdered. I should think Donald Trump Jr, Tucker Carlson, perhaps Jim Jordan or indeed Vance would've been more likely replacements - but with probably worse results in November.