There is something quintessentially American about political bumper stickers. They are blunt, dogmatic, occasionally witty and always provocative. If that's not an apt description of the zeitgeist, I don't know what is.

Unfortunately, in the midst of an election season rich in gawking, there haven't been many bumper stickers to gawk at in my reliably blue neighborhood outside Boston. I fared better on a recent trip to New Hampshire. Pickup trucks rule the rural routes of my swing-state neighbor, and several of their bumpers featured Donald J. Trump stickers.

My children squealed each time they spotted one and began to chant, "Donald Trump! Donald Trump! Donald Trump has a big fat rump!" — a practice upon which I frown, often while smirking.

Outside Derry, my wife noticed a truck on our right. Its rear bumper touted Trump, while the message on its cab window heralded Hillary. We speculated that the vehicle belonged to a couple with opposing loyalties. Drawing closer, we saw that the sticker on the window didn't read "Hillary for President." It read "Hillary for Prison."

My immediate response was a rant roughly as mature as my kids' nursery rhyme. Bumper stickers used to be a cheap and humble means of announcing public support for a candidate. These days, they're a gauge of our escalating political rancor. A slogan from the 1950s, such as "I Like Ike" or "All the Way with Adlai," scans as positively quaint next to a car bumper that roars "Trump the Bitch." The president's iconic 2008 affirmation "Yes We Can" has found a malignant echo in the anti-Trump "Yes, We Klan."

It's not just the rhetoric that has intensified. It's our reaction to it. I remember seeing bumper stickers for Bob Dole back in 1994 and thinking to myself, "He means well, but his policies are too conservative." By 2004, I was routinely pegging Bush-Cheney drivers as macho warmongers. Four years later, I dismissed Romney fans as rich, shortsighted and lacking compassion.

Now when I see a Trump bumper sticker, the judgments I make about the driver are even more detailed and insidious: white, uneducated, aggrieved, racist, misogynist, gun-owning and prone to violence.

I suspect a Hillary sticker automatically generates a corresponding set of slanders within Trumpniks. Whoever's "with her" must be a corrupt gun-grabber, wimp, snob, reverse racist and so on.

It is this pattern of thought — the speed, specificity and seething sanctimony of our assumptions — that travels to the root of our current political crisis.

In 2014, political science researchers at Princeton and Stanford universities tested the divide and concluded: "Hostile feelings for the opposing party are ingrained or automatic in voters' minds. Partisans discriminate against opposing partisans, and do so to a degree that exceeds discrimination based on race."

At Emory University, other political scientists found that Americans are increasingly driven more by a fear and loathing of their opponents than a positive identification with their own candidates. This so-called negative partisanship is not grounded in objections to policies advanced by the other party. It is tribal.

Bumper stickers, then, are tribal markings — emblems of an age that privileges ad hominem over reasoned discourse, that dispatches complex problems with sound bites. In a nation that is increasingly self-segregated into red and blue realms, bumper stickers represent genuine, unplanned encounters with the Other. They remind us, rudely, that ours is not the only tribe in the jungle.

I am not suggesting that bumper stickers are proliferating. In a world ruled by digital self-expression, why bother with something as analog as a strip of vinyl? A Twitter account will broadcast your views in a few keystrokes. Or heck, why not post an emoji on Facebook? Besides, given the vitriol that has dogged the 2016 campaign, voters may not want to advertise their political identities in such an indiscriminate manner.

My wife and I have joked about this. Sure, we want to signal our support for Clinton. But what if a deranged Trump supporter in a Ford F-150 takes umbrage? Nobody wants to make an enemy at 70 miles per hour.

As absurd as this may sound, in the unfungible world beyond our screens, bumper stickers represent a form of commitment. And as ephemera go, they can have a curious staying power.

Recall the passage in Kurt Vonnegut's 1969 novel "Slaughterhouse Five" in which the author mentions that his hero, Billy Pilgrim, has a "Reagan for President!" sticker on his Cadillac. Vonnegut intended this detail to signal his hero's political naiveté, given that Reagan was little more than a fringe candidate in the 1968 race. As it happened, history rendered Billy Pilgrim unwittingly prescient.

It bears mentioning that my aging Honda still proudly (some would say smugly) wears a "Hope, Not Fear" emblem from the 2008 election. In my circles, this is a status symbol, an affirmation that I was an early investor in the Obama legacy. But I should also admit that I possess a less-inspiring sticker from the George W. Bush years. It reads "Impeach the Lying Little ..." — you get the picture.

I fully intended to slap that sticker on my bumper, but I never got around to it. Perhaps I was afraid that Dick Cheney, or one of his fans, would shoot me in the face. But I like to think my reluctance was a bit more principled.

As a means of discourse, after all, bumper stickers are little more than a joy ride. But the raw prejudice they elicit represents a powerful symptom of our historical peril.

Our democracy grants us the right to free speech in all of its outlandish forms. We can and should participate in vigorous public debate about our candidates and our crises of state. But a democracy dominated by bigoted wrath — by the lazy pleasures of bumper-sticker politics — is on a road to nowhere.

Steve Almond's latest book is "Against Football: One Fan's Reluctant Manifesto." He wrote this article for the Los Angeles Times.