Sept. 11 brought America together. Today’s crises are driving us apart.

Like many Americans, Renée Thompson felt an overwhelming sense of national unity in the days after the Sept. 11 attacks. And like many Americans 20 years later, she worries about how distant those feelings seem now.

“I remember everybody having the flags on their cars, everybody was just so patriotic and we were one and united, and now it is so divided,” Thompson, of York County, said on an overcast August afternoon when she visited the Flight 93 National Memorial near Shanksville, Pa.

“You can’t disagree with someone,” Thompson said, lamenting today’s discourse. “If someone disagrees with you, you’re enemies.” She added, “The phrase after 9/11 was never forget — but it does feel like people have forgotten.”

National tragedies often soothe political hostility as a country unites against a common threat, political scientists say. But today’s crises aren’t cooling the temperature. They’re often making it worse.

Just look at the fractured responses to the coronavirus pandemic and the Jan. 6 Capitol insurrection, said Michael Macy, a Cornell University sociologist who studies political polarization.

“Events that in an earlier era would have been almost certain to bring people together, it’s not just that they fail to bring people together, they actually create divisions,” Macy said. “People are legitimately concerned for the stability of democratic institutions.”

» READ MORE: Complete coverage: Remember 9/11, 20 years later

Fights over masks, vaccines, and responsibility for an attack on America’s democratic system have broken down along familiar lines, mapping onto existing political and geographic divides.

More broadly, the country is beset by conspiracy theories and the ease with which people can curate the information they consume, cultural and economic power concentrated among urban, college-educated elites, disproportionate political might for rural areas, declining trust in institutions ranging from religion to newspapers to schools, and sorting that has left the two major parties ideologically lockstep and geographically separate.

There are few conservative Democrats or liberal Republicans. Urban and suburban areas are heavily Democratic, rural regions almost exclusively Republican.

While the Shanksville memorial honors Americans who, it’s believed, prevented an attack on the U.S. Capitol, this year began with a domestic assault on that very center of democracy.

“We’re not talking to each other and we’re not accepting even an agreed source of information,” said Tom Kean, the former Republican New Jersey governor who led the bipartisan commission that investigated the 9/11 attacks. “I’ve never had a time in my lifetime where it’s been like this — never. And I don’t see it getting any better.”

Macy’s research has drawn parallels between political polarization and climate change: There’s a level, he fears, where acrimony gets so bad there’s no way to cool things down.

» READ MORE: The Divided States of Pennsylvania: How one state embodies America's political discord

Polling bears out some of those fears, and the underlying divisions.

Almost half of Americans, 46%, now say 9/11 changed the country for the worse, while a third say it changed for the better, according to a Washington Post-ABC News poll released last week — with liberals far more likely to say it made the country worse. About 80% say the pandemic will have a lasting negative impact, this time with conservatives much more pessimistic.

More Americans are now worried about domestic extremism than they are about external threats, according to an August survey from the Associated Press. And a Pew Research Center poll of 17 countries this spring found Americans the most divided — by far — over coronavirus restrictions.

By contrast, 9/11 brought a moment of political unity, with some significant exceptions, after the fiercely disputed 2000 election. President George W. Bush’s approval ratings skyrocketed. Congress took bipartisan votes to authorize wars in Afghanistan and Iraq and pass the Patriot Act.

Many who supported those steps came to regret it, but the national mood was so singular that few stood in the way. Kean’s commission won support from both parties.

Similar attempts to investigate the Capitol riot have been assailed by most Republicans. Masks and vaccines almost universally recommended by doctors and scientists have met fierce pockets of resistance.

Rudy Giuliani, briefly dubbed “America’s Mayor” after 9/11, has become a caricature of partisanship, leading the parade of false conspiracies about the 2020 election.

Even successes — like the development and distribution of a coronavirus vaccine by two different administrations — aren’t recognized, said Kathleen Hall Jamieson, a University of Pennsylvania professor who studies political communication.

“We may have come to a point where we can’t celebrate the legitimate accomplishments of any administration that is not the administration that we voted for, and that’s sad,” she said.

Still, several experts — and lawmakers who were at the Capitol for 9/11, the onset of the coronavirus, and the Jan. 6 attack — see hope in this grim moment.

Bob Menendez still recalls a Capitol Police officer urging him to “get out of the building!” on 9/11. But Menendez, then a New Jersey Democratic congressman, just as vividly remembers coming back to join fellow lawmakers on the Capitol steps, singing “God Bless America.”

Menendez, now a senator, believes 9/11 was different from the current crises. A foreign threat, he said, prompts a distinct reaction. And the recent challenges came under former President Donald Trump, who actively stoked division.

”There was also no real moment where Trump called for national unity,” said Neil Malhotra, a Stanford University politics and economics professor.

Trump repeatedly downplayed the severity of the virus, mocked safety measures, and spread the lies that drove the insurrection.

A different leader, Menendez said, might have brought America together. He and others pointed to Bush, who stood in the rubble of the World Trade Center and visited a mosque to signal that Muslims were not America’s enemy.

“Leaders do make a difference,” Menendez said. “I didn’t agree with George W. Bush, but I will say in that moment he brought the nation together.”

Pat Toomey, then a Republican Pennsylvania congressman, was preparing for a committee meeting when two planes struck the World Trade Center. He, too, said the current situations are different than a foreign attack that killed thousands in a single morning.

Acknowledging the country’s sharp divides, he said crises still bring common purpose. He pointed to massive pandemic relief bills passed with overwhelming support last year.

“They had enormous flaws and yet people on both sides of the aisle supported policies that they were not comfortable with,” said Toomey, now a senator. “Congress came together and got that done time and again.”

» READ MORE: In tiny Shanksville, where United Flight 93 crashed, 9/11 is ‘part of who we are’

To Malhotra, “The virus is designed to tear society apart, not unite society.”

“There are people who have absolutely no symptoms from the virus and people who die from the virus,” he said, so any policies force one group to make sacrifices for the other. Urban and rural regions, already politically divided, have experienced the virus at different times and in different ways.

“If 9/11 was a prolonged event that happened every single day for months on end, then you would have seen a huge political fracturing over who’s to blame for the prolonged crisis,” said Khadijah Costley White, a Rutgers University journalism professor.

For all the unity after 9/11, not everyone felt the national embrace. Some saw the roots of today’s rancor.

Muslims, and people mistaken as Muslims, faced suspicion, harassment, and violence, and often still do. A Sikh man in Arizona was shot and killed at a gas station. A Texas man went on a shooting rampage, killing two people.

Timothy Welbeck, the civil rights attorney for the Philadelphia chapter of the Council on American-Islamic Relations, drew a line between those incidents and current divisions. He pointed to the subsequent backlash against the country’s first Black president, Barack Obama (falsely said by some on the right to be Muslim); the pandemic-era rise in anti-Asian hatred; and anti-immigrant rhetoric stoked by Trump.

“There are touch points for each,” Welbeck said, “but generally the climate of the country has been more receptive to these types of bigotry and xenophobia, particularly as it relates to more marginalized groups.”

The Capitol attack, he noted, was driven by a false belief that “there was some type of election fraud in cities that were largely Black and brown people.” Some rioters paraded Confederate flags through the Capitol.

Many on the right, meanwhile, see a political and media elite that they believe disdains, dismisses, and tries to silence them.

White, of Rutgers, said Jan. 6 drew a different response than Sept. 11 because it was for a cause supported by a significant share of Americans.

“It was a terrorist attack, but it was in the name of a particular political regime,” she said. “We just haven’t seen the zeal that I think we would have seen if the people invading the Capitol had been brown or Muslim or Black.”

Even those who see differences between 9/11 and today largely agree the country has become more divided.

Political affiliation has become less about policy preferences and more about tribal identity.

“You can compromise on issues,” said Michael Macy, of Cornell. “You want it to be 70, I want it to be 30, let’s settle on 50.”

But when a fight is over identity, he said, “you can’t compromise.”

“It’s either I win or you win,” Macy said. “And we hate you.”

Staff writer Julia Terruso contributed to this article.