Where is the line drawn between immoral inactivity and a simple lack of action?
Under a Kantian framework, inactivity is immoral when it violates a moral obligation you have. This is generally never, since moral obligations are generally negative (i.e. "don't kill someone" is a moral obligation). Kant does posit an ambiguous duty of beneficence, which states that we have a duty to help others. He doesn't say to what extent we have to help others, so it's still unclear if we should vote for Hillary to stop Trump (or vice versa). To be honest, this duty is usually ignored. The main point here, regardless of the duty to help others, is that if Trump or Hillary wins the election, blame can not be placed on any individual who failed to vote. In fact, it's not the voters' fault at all. If Trump does something stupid, only Trump can be blamed.
A utilitarian framework provides a clearer bright line. All inactivity, in the exception of one case, is immoral. It is only moral if any action at all would make the situation worse. The thing is, we have to take into account others' actions here, too, unlike under a Kantian framework. So, if we think Obama is a better president than Hillary or Trump, we should abstain from voting so he can remain president, right? Except, other people will vote for Hillary or Trump, rendering our action moot. So, we have to vote for the "least-worst" candidate under this framework.