The admonition by the Church to vote is best expressed in paragraph 2240 of the Catechism:
Submission to authority and co-responsibility for the common good make it morally obligatory to pay taxes, to exercise the right to vote, and to defend one’s country.
But if you take this to mean one must vote in every election possible, that would be a misinterpretation. For a comparison with another admonition, consider paragraphs 2443-2449 about the love for the poor and the duty to charity. These paragraphs do not imply that we are obligated to give everything we have to the poor. God will read our hearts and discern whether we have shirked our duty to the poor. Similarly with voting, God will read our hearts and determine if not voting in one election was part of a general abandonment of the duty to civic engagement. It is not our place to say if one’s response to any given election is or is not a sin.
We can only cite principles, not rule on every individual case.
I would consider that there are tons of poor people, and arguably tons of taxes.
Elections, especially national ones, are relatively infrequent.
Part of the duty to vote includes not only voting for the better person, but often defeating the worse one. Trump and Clinton, for example, are not equally evil. One is less evil than the other (not saying who, as I do not know or care). You have to vote to defeat the more evil one.
Take our own elections here. Conservative Stephen Harper is no pro-life activist. Liberal Justin Trudeau and NDP Thomas Mulcair have set party policies that require their caucuses to vote for abortion. The Conservative policy is to not legislate to regulate abortion (abortion laws are nonexistent in Canada), but leaves its caucus members free to vote according to their conscience. Further, the Conservative policy is to legislate against euthanasia, while the other two left-leaning parties have either voted to support it or hold it in a positive light.
So in Canada, I have three evil parties, a cowardly right-leaning one and two explicit pro-abortion left-leaning ones. What do I do?
I voted Conservative, because it’s the least evil of the three. It allows conscience votes on abortion and is against euthanasia. Not because I’m a fan of Stephen Harper, but because I needed to do what I could to defeat the two more evil parties. Had I not voted, I would have shirked my duty. My vote lost, but I did my part. My conscience is therefore clear. Had I not voted, I would have been complicit in Trudeau’s win. Despite my loss, I am not.
No two candidates are equally evil. One is always less evil than the other. Therefore there is never a reason to withhold a vote, even if just to try to defeat the worse one.