Ramesh Pannuru and Robert P. George published a helpful article at National Review explaining why the issue of abortion is enough to scuttle any desire to vote for Biden (see this video to hear Biden's whole-hearted commitment to abortion being the "law of the land.").
Below are some quotes from Pannuru and George's piece, but the whole thing is really worth reading (or listening to).
What the Church teaches is that every member of the human family — irrespective of race, sex, or creed, but also, and equally, irrespective of age, size, stage of development, location, or condition of dependence — is the bearer of inherent and equal dignity and, as such, is entitled to legal protection against violent assault.
For government to permit abortion, the Church teaches, is for government itself to commit an injustice against its victims—denying a disfavored class, the unborn, protection it affords to all others. To be responsible, or partially responsible, for the injustice of the law in exposing unborn children to legally authorized lethal violence is to be complicit in grave injustice.
though abortion and slavery differ in many respects, they are alike in not admitting of the option of the “personally opposed, but . . .” position. The reason to be against slavery — its radical denial of the equal dignity of the victim and thus its grave injustice — is the reason for prohibiting it. It is precisely the same with abortion.
while citizens have other responsibilities, too, the injustice of abortion has a gravity that means it must be weighed more heavily than ordinary political issues, even important ones
Biden has expressly pledged to deny appointment to the Court for anyone who does not support what amounts to an unlimited right to abortion.
If one acknowledges the gravity, scale, and scope of the injustice of abortion, and of a legal regime that denies to an entire class of human beings the most basic of human rights, thus exposing them to lethal violence, then it is hard to imagine what proportionate reasons there could be for joining one’s will to the desire of a supporter of it
They do not argue that a Christian (or person concerned with ethics) must vote for Trump. That is more of a prudential issue.
You might also find this article from John Piper on why we are all "one issue" voters helpful (written 25 years ago).