Typically they are flawed for the same reason the “personally opposed” position collapses into incoherence: They ignore the particular injustice of treating an entire class of persons, unborn children, as nonpersons with no right to life. That’s the main (though not the only) thing wrong with arguments that other issues — climate change or health care, for example — should be given weight equal to that given to abortion. Whoever is right about what health-care arrangements are best, the advocates of none of them argue for the direct killing of, or the permission to kill, large numbers of people because they belong to a disfavored class. It’s what’s wrong with the argument that overturning Roe won’t prohibit abortion by itself: In addition to being (as a practical matter) a prerequisite for prohibiting abortion anywhere in the U.S., overturning Roe will definitely accomplish the goal of excising a vicious anti-principle read into our fundamental law (with no warrant in its text, logic, structure, or historical understanding) by judges who acted in a grossly prejudiced manner against unborn children. It’s what’s wrong with a focus on the abortion rate to the exclusion of abortion law. . . .
On a somewhat different plane are arguments that abortion must be weighed against other moral evils. Several such arguments have been made to dissuade pro-lifers from voting for President Trump. He supports the death penalty, which a significant number of opponents of abortion consider to violate the sanctity of human life. He has advocated war crimes. Worst, he imposed a zero-tolerance policy at the border that had the predictable effect of separating large numbers of children from their parents. He and some of his aides reportedly welcomed this humanitarian catastrophe as a deterrent.
There are three things to consider when evaluating such issues. The first is the gravity of the evil. Even if it is wrong to execute someone judged guilty of murder even in a fair trial (as we believe), it is not on the plane of injustice of killing the innocent, nor does the Catholic Church regard it as such. The second is the scope of the evil. Abortion kills nearly 1 million unborn children each year in the United States; executions are less common than death by lightning strike; Trump, for all his many faults and failings, has not followed through on his disgusting bluster about killing terrorists’ relatives. And the third is that an argument against voting for a candidate who opposes abortion is not an argument for voting for a candidate who supports it. If both leading candidates for an office fail a threshold test of moral acceptability, it remains possible to choose an independent or third-party candidate, write someone in, or abstain.
In other words, abortion is an act of violence and injustice against a whole segment of human beings. In this, it is like slavery. You can’t be “personally opposed” to slavery, while still believing that it should be legal. I would add that while the violence of abortion is committed by abortionists, the injustice of it all is perpetrated by the government that allows it. This is also like slavery: not everyone owned slaves, but the laws that allowed it and the government that protected it were at fault and needed to be changed.