President Obama, joined by Brittany Packnett, of the President's Taskforce on 21st Century Policing, at a White House meeting Wednesday. (Carolyn Kaster/AP)

“We are not as divided as we seem,” President Obama said at an interfaith service in Dallas for five officers slain by a gunman. His predecessor, George W. Bush, sat a few feet away, on a stage that featured blacks and whites, citizens and police officers, a Methodist reverend, a rabbi and an imam.

“At times, it seems like the forces pulling us apart are stronger than the forces binding us together,” Bush said.

But words may not be enough to overcome those fraying forces. This has been a violent and tragic summer, and it follows a period of pronounced divisiveness in American political life. The promoters of national unity may be outgunned, both metaphorically and literally.

Tens of thousands of people will be arriving this weekend in Cleveland for what is poised to be a combustible Republican National Convention. Dozens of groups from across the political spectrum are expected to stage street protests. Water pistols and toy guns will be prohibited, but real guns will be permitted under Ohio’s open-carry law. Wags call this convention the Trumpocalypse.

The sense that America is more divided than it used to be is backed by hard data. There’s been a sharp spike in the contempt that partisans express for their opponents, according to Pew Research Center polling.More than 4 in 10 Democrats and Republicans say the other party’s policies are so misguided that they pose a threat to the nation.

On many racial issues, whites and blacks see different realities. For example, African Americans were far more likely than whites to disapprove of grand jury decisions not to indict police officers in the deaths of Eric Garner and Michael Brown and of George Zimmerman’s not-guilty verdict in Trayvon Martin’s death.

A separate Pew poll this year showed that 88 percent of blacks think more needs to be done to give blacks equal rights, while 53 percent of whites agreed. A recent CBS News-New York Times survey found that about half of white Americans but three-quarters of African Americans say police are more likely to use deadly force against a black person than a white person.

Barack Obama gained national fame in 2004 when, as a U.S. Senate candidate, he gave a speech at the Democratic National Convention and talked of common American values:

“Now even as we speak, there are those who are preparing to divide us, the spin masters and negative ad peddlers who embrace the politics of anything goes. Well, I say to them tonight, there’s not a liberal America and a conservative America; there’s the United States of America. There’s not a black America and white America and Latino America and Asian America; there’s the United States of America.”

After his election in 2008, President Obama was not alone in believing Americans could come together. More than three-quarters of the public thought Obama would unite the country rather than divide it, according to a CNN poll. Six years later, a Washington Post-ABC News poll found a 55 percent majority saying Obama had done more to divide the country than bring it together.

“It’s one of the few regrets of my presidency — that the rancor and suspicion between the parties has gotten worse instead of better,” Obama said in January in his final State of the Union address.

Historically, Americans have come together, at least briefly, in times of crisis. There has been minimal evidence of that in this terrible summer of violence. America has experienced the worst mass shooting in its recent history as well as the most lethal attack on law enforcement since 9/11. Americans have also witnessed, in raw video feeds, the deaths of Alton Sterling in Louisiana and Philando Castile in Minnesota, both shot by police officers.

After the Dallas attack, Trump initially used measured, statesmanlike language: “Our nation has become too divided. Too many Americans feel like they’ve lost hope. Crime is harming too many citizens. Racial tensions have gotten worse, not better. This isn’t the American Dream we all want for our children,” he said in a written statement released the morning after the five officers were killed.

He shifted within days to more-incendiary rhetoric. Speaking of protests by Black Lives Matter activists, he said several times, without providing any evidence, that he’d seen people asking for a moment of silence in honor of the gunman who murdered the Dallas officers.

Hillary Clinton on Wednesday spoke in Springfield, Ill., the site of Abraham Lincoln’s historic “House Divided” speech of 1858. She said there is too much violence and hate in America and too little trust and common ground. And she turned confessional:

“As someone in the middle of a hotly fought political campaign, I cannot stand here and claim that my words and actions haven’t sometimes fueled the partisanship that often stands in the way of progress. So I recognize I have to do better, too.”

One common argument in academic circles is that polarization and distrust have been intensified by the Internet, which is awash in misinformation, funnels people into echo chambers, and provides forums for anonymous hate speech.

But the mainstream news media is hardly innocent when it comes to national divisiveness, because journalists typically find conflict more interesting than harmony. “There’s a dominant polarization narrative that is driving coverage,” said Kathleen Hall Jamieson, director of the Annenberg Public Policy Center at the University of Pennsylvania.

Research on “cultural cognition” indicates that people are tribal when it comes to certain issues, such as climate change, gun control, abortion and evolution. People tend to trust the news sources that confirm their beliefs. Those beliefs become statements of identity and community loyalty. When Trump said, “I am not a great believer in man-made climate change,” he was branding himself.

President Obama followed his Dallas appearance with a White House event in which he intentionally seated police officers and community leaders next to Black Lives Matter activists. Dubbed the “White House Convening on Building Community Trust,” the gathering featured the president, with jacket off and shirt sleeves rolled up, performing the role of discussion leader.

Later, Obama posted a summary of the event on Facebook, urging the public to have conversations and find solutions: “That’s the path out of moments like these. Not to withdraw, or shout each other down, but to reach out to each other — even if it’s difficult — and find some common ground.”

But common ground may be unreachable in some cases. Activists with legitimate grievances demand to see changes and social progress before they stand down. Many people feel that their values are under attack and need to be defended vigorously.

And some political leaders and media figures prefer to fire up their ideological bases rather than seek harmony or unity. Divisiveness can be a strategy or even a business model.

“They’re conflict entrepreneurs. They kind of thrive on the pathologizing of our politics,” said Dan Kahan, a Yale professor of law and psychology who studies cultural cognition. “They’re a problem. But most people don’t have an appetite for this. Most people don’t want to ram their values down other people’s throats. They just want to put food on the table.”

The president kept pushing as the week went on — and on Thursday participated in an emotional town hall discussion on race relations, broadcast on national television in prime time.

“Nobody’s more hopeful than me,” Obama said. “I’m — I’m — I’m Mr. Hopeful when it comes to these issues. I’ve said from the start we’re not as divided as we seem, and I think we’re going to solve it.”