1. It's "doctrine", "principle", or "rule" of double effect, not "law.  
2. The first paragraph is not an example of DDE but, at best, an accident (at least insofar as it is written).  The author's explanation of DDE in the second paragraph is tortured and erroneous (to oversimplify; of course one may be "aware" that a bad result may be consequent to a good one; "awareness" of the bad is not a reason to postpone or omit a "good).
3. The author cannot demonstrate that his fears of the bad effect are "foreseen" or even "foreseeable", if unintended, by those whom he seems to berate as single-issue voters.

Let me be clear. I didn't vote for Trump (or Hillary) and I'm not stumping for those who may have; my comments are to the content of the article only and its application to the situation.  D+, professor, start over.  Extra credit if you add the consequentialist or utilitarian viewpoint.