Even before the assassination of Charlie Kirk, an influential right-wing activist, there were signs of a looming political crisis. Rising polarization and the coarsening of public discourse left little room for shared understanding. Acts of violence, targeting figures on the left and the right, had begun piling up.

But the killing of Mr. Kirk on a Utah college campus on Wednesday — shortly after he began speaking to a young crowd on a sunny afternoon — raises the possibility that the country has entered an even more perilous phase.

On social media, it was easy to find left-wing posters reveling in Mr. Kirk’s death and suggesting he got what he deserved. On the right, initial expressions of grief and shock were overtaken by open calls for political reckoning and vengeance. There were ominous proclamations that the country was on the brink of civil war — or should be.

The outbursts worried experts, who warned that Americans’ tolerance for politically motivated attacks has been growing at a striking pace.

“We’re basically a tinderbox of a country,” said Robert Pape, a political science professor at the University of Chicago who has been conducting regular polls to measure attitudes toward political violence since supporters of President Trump attacked the Capitol on Jan. 6, 2021. “We are seeing more radicalized politics and more support for violence than at any point since we’ve been doing these studies in the past four years.”

The shooting of Mr. Kirk, 31, was captured from multiple angles on video; gruesome footage of blood spurting from his neck quickly went viral. A few days earlier, many Americans had watched similarly disturbing footage of a young public transit rider in Charlotte, N.C., who was stabbed to death by a stranger in an unprovoked attack.

That killing had become entangled in an escalating national debate over Mr. Trump’s desire to send the military into Democratic-led cities to combat crime. In a country where the president calls his opponents “scum,” and opponents accuse him of fascism, it already seemed to many that the fabric of public discourse had hopelessly frayed.

Mr. Kirk, who was prolific on social media, was himself deeply engaged in the conversation about crime, posting on X just hours before he was shot that it was “100% necessary to politicize” the Charlotte murder.

“I think that you have a cultural civil war underway,” said Newt Gingrich, the Republican former House speaker, in an interview on Wednesday afternoon. Mr. Gingrich said he fully supported Mr. Trump’s efforts to upend the American status quo. But he acknowledged that they were rocking the ship of state.

“You have very profound differences about the very basics of life,” he said, referring to partisan divisions. “And the country has not figured out how to sort it all out yet. We felt like we were under enormous pressure from the Obama-Biden cycle. The left feels like it’s under tremendous pressure from the Trump cycle. And we don’t know how this is all going to play out.”

Among those studying the public’s appetite for political violence — a fast-growing discipline in American academia — the mood on Wednesday was grim.

In Dr. Pape’s most recent survey in May, nearly 39 percent of Democrats agreed with the idea that removing Mr. Trump from office by force was justifiable. At the same time, nearly a quarter of Republicans said it was justifiable for Mr. Trump to use the military to crack down on protests against his agenda.

Garen Wintemute, a physician and the director of the Violence Prevention Research Program at University of California, Davis, argued that a spiraling cycle of violence is not a foregone conclusion.

“The task we face now is to not let the people at the extremes pull the rest of us over the edge with them,” Dr. Wintemute said. “We need to make our rejection of political violence clear.”

Mr. Kirk was a committed partisan. In the immediate aftermath of the shooting, Matthew Dowd, a political analyst on MSNBC, called him a “divisive” figure who had engaged in “hate speech.”

“You can’t stop with these sort of awful thoughts you have and then saying these awful words and not expect awful actions to take place,” Mr. Dowd said on the air. “And that’s the unfortunate environment we are in.”

Rebecca Kutler, the MSNBC president, called Mr. Dowd’s comments “insensitive and unacceptable.”

At first, many right-wing commentators expressed their shock through calls for prayer. Mr. Kirk was a Christian, and many popular influencers share his faith. Benny Johnson, a podcaster who in the past worked at Turning Point USA, the organization co-founded and run by Mr. Kirk, called on his followers to “get down on your knees and pray.”

But after news emerged that Mr. Kirk had not survived, Mr. Johnson’s tone, along with that of many others on the political right, took a decidedly dark turn. Mr. Johnson called television news anchors “demons,” proclaimed that he was “burning with righteous anger” and said that Mr. Kirk was a martyr.

On conservative Fox News, Jesse Watters, the popular prime-time TV personality, spoke passionately about the attack, and the need to somehow strike back.

“We’re sick, we’re sad, we’re angry, and we’re resolute, and we’re going to avenge Charlie’s death in the way Charlie would want it to be avenged,” he said on Wednesday.

Mr. Watters listed a number of threatening or violent acts perpetrated in recent years by the people on the left: the armed man arrested in 2022 who wanted to kill the Supreme Court justice Brett Kavanaugh. The shooting of two Israeli Embassy aides in Washington in May. The vandalizing of Teslas to protest Mr. Trump’s sometime ally, Elon Musk. The 2017 shooting of Representative Steve Scalise, a Louisiana Republican.

“Whether we want to accept it or not, they are at war with us!” Mr. Watters said. “And what are we going to do about it? How much political violence are we going to tolerate?”

Federal and local authorities have not yet identified a suspect in the shooting, yet the far-right activist Laura Loomer, with no evidence, called it a “professional hit.”

Matt Forney, a right-wing journalist known for racist and misogynistic content, called Mr. Kirk’s assassination “the American Reichstag fire,” alluding to the 1933 fire at the German Parliament building that was used by the Nazi party as a pretext to suspend constitutional protections and arrest political opponents.

“It is time for a complete crackdown on the left. Every Democratic politician must be arrested and the party banned,” Mr. Forney wrote on X.

Alex Jones, the proprietor of the conspiracy theory, posted a video online declaring, “Make no mistake — we are at war.” And Chaya Raichik, a right-wing internet celebrity best known for her popular Libs of TikTok account on X, posted similarly, “This is war.”

On Patriots.win, a far-right website where some of Mr. Trump’s most fevered supporters have gathered for years, the violent language included posts like, “Start the Democrat extinction event.”

Still, one prominent right-wing figure, Nick Fuentes, a notorious racist and antisemite, beseeched his followers to be calm amid the persistent calls for violence.

“The violence and hatred has to stop,” he wrote. “Our country needs Christ now more than ever.”

Ruth Braunstein, an associate professor of sociology at Johns Hopkins University who studies political violence and polarization, said she was concerned that the slaying of someone she described as a “pivotal figure” on the American right could mobilize groups that have been waiting for just such a catalyst.

“The right, she said, “has well-organized and trained groups, including militia organizations, that are basically waiting for a moment to be called into action in defense of what they view as the nation.”

She added, “All it will take is the slightest hint from the political leaders, including the president, but also anyone else, that this is the moment that they’re needed.”

Though Mr. Trump has engaged in the most incendiary rhetoric of any president in recent memory, his initial reaction to the news was restrained. He ordered flags across the country lowered to half-staff until Sunday.

On Truth Social, he praised Mr. Kirk as “legendary” and offered his sympathy to his wife and family.

Later, though, Mr. Trump blamed Mr. Kirk’s murder on the news media and the “radical left” for “demonizing those with whom you disagree.”

“For years, those on the radical left have compared wonderful Americans like Charlie to Nazis and the world’s worst mass murderers and criminals,” Trump said. “This kind of rhetoric is directly responsible for the terrorism that we’re seeing in our country today, and it must stop right now.”

In the Rio Grande Valley on Wednesday, Sergio Sanchez, a longtime conservative radio host and former chair of the Hidalgo County Republican Party, spoke through tears as he accused some liberals of “perpetuating and promoting this culture of hate and violence.”

Mr. Sanchez also pined for what he believed was a better and simpler chapter in the American story.

“I’m a kid of the late ’80s,” he said. “I remember a time in America when we would live and let live. And I don’t recognize our nation anymore.”