The single issue that matters in this presidential election is whether Donald Trump is capable of being president of the United States. Any voter who believes he is not must vote accordingly. That means voting for Hillary Clinton.

It is a difficult conclusion for most Republicans and many Bernie Sanders supporters. They recognize the dangers of Trump’s rhetoric, and they do not embrace his positions. They can acknowledge why he should not become the leader of the free world.

A popular “compromise” has emerged: refuse to vote for any president at all. Or, vote for a third-party candidate. Or, just write someone in. Chuckle, chuckle.

This conflict is one that deserves sympathy. This disappointment in a choice that feels like no choice at all should not be belittled. Unfortunately, this decision cannot be driven by such emotions.

Anyone who does not endorse Trump must rally around the greater cause and bigger message this election has come to symbolize. The majority of American voters must unify around a principle, if not a candidate. Trump’s vengeful, intemperate, and polarizing world view is not representative of our collective will.

For better or for worse, the only way to reassure ourselves and the world of our common decency, our common humility, and our common pursuit is to elect Hillary Clinton.

She is not the perfect candidate. For some, she is from the “wrong” party. For others, she is too one-dimensional, an automaton so of the political machine that she does not appear to exist outside of it. She can be accused of saying whatever it takes to win. Her name is synonymous with scandals that are now shorthand: Monica Lewinsky, Benghazi, emails on a server in Chappaqua.

But Trump can be condemned for the same or similar shortfalls.

He has wanted to be president in enough election cycles that he’s toyed with it as a nominal Democrat and Independent, only to land this time with the Republicans. He is a narcissist, obsessed with portraying himself as a winner in any context. He is known for corporate bankruptcies and failed business ventures, high-profile marriages and divorces, sexualized comments about his daughter, and impulsive/compulsive tweeting.

When Trump is subjected to the same criticism that nips at Clinton’s heels, his Achilles is equally exposed.

On the other hand, Clinton has a resume that would be taken as indisputable evidence of her qualifications if she were applying for any other job. Her unalloyed commitment to public service should be respected, not sneered at. She has a track record that indicates what we’ll get if we elect her.

True, we’ll likely get more of the same, and some don’t want more of the same.

But change is only good when the change itself is for the good.

Trump promises change, but it comes as advocacy for policies and practices that embarrassed us decades and centuries ago, and from which we have worked hard to evolve. If we can predict anything of Trump, it is that he will be frighteningly unpredictable.

His decisions, his words, his actions, his reactions, will correlate to the degree of praise he receives as a prompt. When provoked, he has proven and proclaimed that he will respond without regard to the restrictions of politics, the law, proportionality or humanity. No argument is beneath him, no victory is too empty, no tactic is too ruthless.

This is not a choice between her and him. This is a choice is between the familiar and the unfathomable.

The presidency is not a lark, or an experiment, or written in erasable ink. We cannot contribute to a potential Trump victory by refusing to support his only potential vanquisher, especially where she is at worst what we expect.

It is a mathematical and practical reality that Trump will not be defeated by a blank space, or the Green Party candidate, or the Libertarian candidate, or the friend you write in as a joke.

But he must be defeated.

Perplexed Republicans and inspired Sanders supporters, vote for the person you don’t love to defeat the person you fear.

Vote for Hillary.

Imagine what could happen if you don’t.

Abby Diaz grew up in Falmouth and lives there again, because that’s how life works. She blogs at whatsleftover.com. Follow Abby on Twitter: @AbbyDiaz1.