Go ahead, throw your vote away: a math lesson for critics of third-party voters.

• Author: Hazlett, Thomas W.

I recently had a pleasant encounter with a great and outspoken American who, despite his libertarian leanings, supports Hillary Clinton for president. I congratulated him on making a tough call but allowed as how I was looking forward to casting my ballot for the Libertarian Party's flag bearers, Gary Johnson and William Weld. "It will be unadulterated pleasure," I offered, "as there is no opportunity cost."

My correspondent fired back: "Opportunity cost is Trump gets elected."

I stand by my recklessness.

Here's where the curious nature of the American Electoral College comes in handy. Even where my vote--or the votes of my 100 closest, most easily influenced "inner circle"--might swing an election, there is simply no real chance that pushing either Donald Trump or Hillary Clinton over the top in South Carolina, where I live, will determine the outcome of the presidential race. If Hillary wobbles to victory in my current state of residence, she would have already demolished The Donald in the Electoral College. Similarly, in Maryland (where our family lived until 2014), a squeaker for Mr. Trump would indicate that Ms. Clinton had been vanquished in a yuuuuuge land-slide elsewhere.

Now, it is extremely unlikely that any one person's vote will rock even one state's electoral outcome. In the closest state presidential election of the last half-century--New Mexico (no, not Florida) in 2000--the final margin for Al Gore came to 366. And even that did not swing the national prize.

But set those slim odds of individual influence at the state level off to one (long-shot) side. Assuming that you live in a red or blue, and not a purple, state, you swing completely out of the loop. In its most recent election forecast, the prediction site FiveThirtyEight estimates that there is a 17.9 percent chance that Florida will decide the election (putting one of the candidates "over the top"). Next in line are Ohio and Pennsylvania, where the chances are 11.5 percent each, followed by Michigan at 8.7 percent and Wisconsin at 6.2 percent. When you account for North Carolina, Virginia, Colorado, Minnesota, Georgia, Nevada, and Iowa, you've eliminated all the states with as much as an estimated 2 percent chance to determine the outcome. Multiply that by the probability that one's own vote can throw one's state from Hillary to Donald or back, and the prospect that your vote will crown the next chief of state is neatly forecast as equal to 0.0.

This safe harbor...