I think Walker significantly (and strategically) overestimates the amount of moral angst amongst evangelical Trump supporters. But many pro-life voters (including myself) will face the dilemma he describes in the upcoming presidential election.
Walker is making the following claim: If you think abortion is a matter of life or death, then you must support whoever opposes it most vigorously, even if he or she is an immoral lout.
There are several responses:
First, it is a moral claim without a limiting principle. It would justify the argument: If you think abortion is a matter of life or death, then you must support whoever opposes it most vigorously, even if he or she suspends the Constitution and rules by decree. Or, even if he or she is sympathetic to chattel slavery. This should raise immediate ethical flags. The principle cries out for qualification.
Second, the statement contains a false premise. Voters are not choosing a dictator who would have the immediate power to outlaw abortion. They are choosing a politician who may or may not be involved in influencing a public debate that may or may not result in the choice of a Supreme Court justice who may or may not be part of a majority opinion that may or may not overturn Roe v. Wade, which may or may not significantly reduce the number of abortions. A vote for a politician is only tenuously related to a change in the social and legal status of abortion in the United States. Indeed, the number of abortions trended downward in all but one year of the Clinton and Obama administrations.
Third, voting for a candidate is also related to other moral matters of public importance. This is the truest source of complexity: weighing likely political and social outcomes. One’s vote may discourage racism, or result in more empathetic treatment of the poor, or maintain high standards of public ethics, or encourage the proper treatment of women. So, voting for a pro-life president who promises to jail his or her enemies would be immoral. He or she would have the immediate power to destroy the rule of law and almost no immediate power to end abortion. Voting for a pro-life president who treats migrant children and fleeing refugees as so much human refuse would be immoral. Such dehumanization would have immediate consequences. Here the relative value (and urgency) that we assign to various moral imperatives matters greatly.
Fourth, pro-lifers in the United States are going to win the abortion debate only if we persuade enough people to join our side of the argument. We are not going to prevail by gaining power and imposing our view. We can make changes at the margin using legislative majorities and executive power, but these are very marginal. Ultimately, we have a fundamentally persuasive task, requiring us to think about how our arguments look to people with different views. To have those arguments associated with Trump — and thus with misogyny, racism and xenophobia — is not likely to be helpful.
Finally, Christians have an additional burden in this debate: They need to act in ways that do not undermine the reputation of the Gospel. This commitment needs to figure heavily in the weighing of social outcomes.
Given these points, it would be possible for pro-life citizens to vote for a pro-choice candidate under limited circumstances — particularly when the social threat they oppose with their vote is more immediate than the long-term influence of their vote on the number of abortions.
It would be difficult for a pro-life citizen to be an enthusiastic and loyal Democrat, even if my case is correct. But it is possible to imagine circumstances in which voting for a Democrat would be preferable to endorsing immediate harm to the country by a Republican. And we are in exactly such a circumstance.