Every day, I run into Republican friends who can’t stomach a vote for Donald J. Trump but don’t know what to do. Vote for Hillary Clinton, who has trouble with the truth, wants to raise taxes and opposes free trade with Asia? Vote for the Libertarian candidate, Gary Johnson, an outlier who once ran a marijuana business and embraces isolationism? Or not vote at all, maintaining a certain purity but allowing others to decide the next president?

I faced exactly these choices myself. I have voted for every Republican nominee for president since 1980, but I will not this time. Mr. Trump’s appalling temperament renders him unfit to be president, and his grotesque policy formulations mock the principles of liberty and respect for the individual that have been the foundation of the Republican Party since Abraham Lincoln.

Even before Mr. Trump entered the race, I saw this coming. I worked to open a pathway for an independent — a solid third candidate who would attract the votes of the roughly two-thirds of Americans in the center. A serious contender would force the two major-party candidates to compete for votes in the middle, rather than appealing to the wings. I spent a year and a half on the project, but a month ago threw in the towel.

The deck is stacked by the parties against anyone but a Republican or Democrat. An independent has to run an expensive gantlet to gather enough signatures to get on the ballot in all the states, suffers a severe disadvantage in fund-raising, and is effectively barred from the fall presidential debates by a commission loaded with party stalwarts.