There was a category of people [not me, fwiw] for which abortion was the singular issue that made them vote for what they considered a deeply flawed candidate. Everything else paled in comparison.
From the pro-life point of view, it also affects the unborn child who is murdered. For many of them, abortion is literally the same thing as letting people kill their toddlers because they decided that they don't want them for whatever reason. This is the divide.
(I realize that there are other pregnancy complications that don't literally equate to having an inconvenient toddler, and pro-lifers aren't as consistent on how they view those cases, but those are a small minority of abortions.)