If you had asked me four years ago to concoct the most dispiriting and debilitating 2016 presidential campaign, I might have said, “Start with a political family; find a scandal-scarred creature of Washington addicted to 20th-century identity politics.”

“Now find a vacuous outsider; somebody who reflects the worst of modern politics and culture. A celebrity would be perfect. Better yet, a reality star who is famous for being famous, a social media whore, a boor, a bully who traffics in old hates via new technologies.”

I might have picked Hillary Clinton vs. Donald Trump. What could be worse for a creaky, cancerous political system than what the Democratic and Republican parties are brewing up? Nothing really. This is as bad as it gets.

In contests Tuesday that put Clinton and Trump on the verge of a general-election face-off, voters across the spectrum signaled displeasure with the duopoly’s work. Among GOP voters, 37 percent said they would consider a third-party candidate if faced with a Trump vs. Clinton matchup. Democratic voters found socialist Senator Bernie Sanders of Vermont to be more trustworthy than their likely nominee.

More broadly, 53 percent of Americans disapprove of Clinton, according to Gallup, and 63 percent have a negative opinion of Trump. Most voters don’t find either candidate to be particularly honest.

As Michael Barbaro wrote for The New York Times, “Should they clinch the nomination, it would represent the first time in at least a quarter-century that majorities of Americans held negative views of both the Democratic and Republican candidates at the same time.”

Both major parties must now confront the depth of skepticism, resistance and distaste for their front-runners, a sentiment that would profoundly shape a potential general election showdown between Mr. Trump and Mrs. Clinton.

They are devising appeals that are as much arguments that their all-but-certain opponent would be disastrous for the nation as they are messages trumpeting their own virtues or character.

Aides to both predict that a Clinton-Trump contest would be an ugly and unrelenting slugfest, as she pounces on his business practices and personal integrity, portraying him as unscrupulous robber baron, and he lacerates her over ethical lapses and sudden riches, painting her as a conniving abuser of power certain to be indicted in a federal investigation.

There is, both sides concede, plenty of material to mine, stretching back to 1980s Arkansas (for her) and 1970s New York (for him).

This is not to suggest equivalence: The candidates are not equally revolting. But for millions of voters, today begins a process driven by their aspersions toward one candidate rather than their aspirations for another—the acceleration of a grim trend that political scientists call “negative partisanship.”

“Come November,” voter Ed O’Malley tweeted me in response to Barbaro’s story, “I’ll vote for one or the other then go outside and throw up.”

What about people like him who claim to hate their choices and yet consistently vote Democrat or Republican? Imagine a doctor telling you that because of some gnarly disease, he had to cut off one of your arms. You get to choose which one. While your decision would be easy—“I’m right-handed, Doc. Cut off my left arm”—you wouldn’t be happy with your choices.

My friend Matthew Dowd, a former political consultant who now works for ABC News, said Tuesday’s results show just how “corrupt and broken” the political system has become. Even if the most experienced and, arguably, most qualified candidate wins in November, Dowd said via email: “Hillary won’t be able to govern, and the GOP is past its expiration date. Democrats are closing in on theirs.”

A column like this will trigger torrents of manufactured outrage and exaggerations. From the left: How dare you compare Clinton to that bigoted, bullying empty suit of a man? And from the right: ARE YOU NUTS? She’s not qualified! She’d destroy America!

Together, blindly loyal and satisfied partisans represent a fraction of the electorate. Millions of other Americans will suffer through another ugly campaign before making two decisions.

First: Do I even bother to vote?

For those who do cast a ballot, there is the even sadder choice: Which candidate do I loathe the least?