When I tell my Republican friends or mention on Facebook that I may not cast a vote for president or may cast a protest vote for Libertarian candidate Gary Johnson, the immediate response is "not voting or voting for a third party is a vote for Hillary Clinton." That is not true. Obviously it is not factually true. Not voting is not voting and a vote cast of Gary Johnson is a vote cast for Gary Johnson, but I know what they mean. They mean that by not voting, I am denying a vote to Donald Trump, but that assumes I would be voting for Trump if I were voting. Saying a vote for a third party is a vote for Hillary, also essentially is making the same assumption. That is most likely a good assumption because if I had a gun put to my head and were forced to vote and forced to vote for one of those two bad choices, then as of today, I would reluctantly, while holding my nose, vote for Trump.
The not-voting-for-Trump-is-a-vote-for-Hillary argument assumes that my vote is important. It assumes that my vote affects the outcome. It does not. I am not surprised that low-information voters don't understand this but even informed people will make this argument. For it to matter whether I vote for Trump versus not voting or casting a third party or write-in vote, assumes that whoever gets the most votes for president gets elected president. That is not the way it works. Our process of electing the President reflects the nature of our form of government. We are a democratic republic not a popular democracy. We do not have national elections for President; we have 50 state elections for president.
I am virtually certain that Trump will overwhelmingly carry Tennessee. In the Republican primary, he carried every county except one. Something terribly dramatic would have to happen for Tennessee to vote for the Democrat nominee. Polls are pretty reliable and we can know Trump is going to carry Tennessee before the votes are ever cast. Since I am certain Trump will carry Tennessee and since Tennessee's system of assigning electoral votes is "winner take all," I can safely vote third party, cast a write-in for Marco Rubio, myself, or whomever or just vote down ticket and skip voting for President. The national popular vote is irrelevant. If Trump carries Tennessee, he gets all of Tennessee's 11 electoral votes. My not voting or voting for someone other than Trump does not change that outcome.
Having the luxury of not voting for Trump without it helping Hillary would not apply if I lived in some other states. If I lived in a swing state then the effect of not voting for Trump would be to help Hillary get elected. Each state's law determines how that state's electors are selected, and which candidate they vote for. Most states are like Tennessee and "winner take all," but some are not. Some states require that all but two of their electors vote the way each separate congressional district votes. If your state had that district system and you live in a swing district then your vote would matter. If you live in a state that assigns votes by district and your district might vote for Hillary, then the effect of not voting for Trump is to help Hillary's election. Several states have adopted a system that requires the state's electoral votes to be cast for the person who gets the most popular votes nationwide regardless of who carries their state. In that case, your not voting for Trump has the effect of helping Hillary's election.
Some people say that in choosing to vote for Trump over Hillary, they are voting for the lesser of two evils. I understand. If I lived in a swing state or one of the states that have adopted the popular vote system of assigning electors, then I would vote for whom I perceive to be the lesser of two evils and vote for Trump and hope I had made the right decision. Voting for the lesser of two evils is still evil however. Since I have the luxury of my vote not having an impact on the election outcome, I am not going to vote for either of the two evils this election.