I will not vote for Donald Trump and am very willing to vote for Hillary Clinton to help keep him from becoming President. In my life as a political science professor, such an opinion is barely worth stating as it is more or less assumed. In my life as an evangelical Christian (they are actually the same life, I live both of them at the same time), this surprises some people. Given one known Supreme Court vacancy and the distinct possibility of more in the next four years, why vote for Clinton who will assuredly nominate justices who are pro-choice [or insert your favorite Supreme Court Issue here]?
First, because past experience indicates that voting for a President is a very indirect and rather ineffective way of changing the Supreme Court's position. In the early 1990s, a Supreme Court consisting of eight Republican appointees and one Democratic appointee voted to uphold Roe v. Wade 6-3, and the Democratic appointee was not among the six supporters. Further, I have even less confidence in Trump's actual commitment to being pro-life or other issues evangelicals care about than any Republican nominee in my lifetime. I am not a one-issue voter.
Second, because when I put all the other issues on the scales, they clearly outweigh the Supreme Court issue. I don't want to validate Trump's comments about women or minorities, I don't want someone with his temperament representing our nation abroad, and I certainly do not want him as commander-in-chief of our armed forces with access to nuclear codes. In sum, his character is unacceptable and his actual policy proposals strike me as consistently wrongheaded.
Many people I talk to are planning to write in or vote third party. I can respect that position, but realistically either Trump or Clinton will be president. If there is even a chance of the former, the most effective way to stop him from being elected is to vote for Clinton. That, to me, is more important than "sending a message." The pollsters will figure out than many Clinton votes are in part anti-Trump votes anyway.
Trump is making outlandish promises and proclamations and would surely disappoint his supporters who think that America's problems are sufficiently superficial that electing an angry outsider is enough to fix them. Trump is more a symptom of what is broken than the real cause of the brokenness. My hope is that his loss will be a time to think deeply about the underlying issues that brought us here.