Today in the United States, sweeping majorities of the public say they support fair housing laws and the ideal of integrated schools. Nine in 10 say they would back a black candidate for president from their own party, and the same say they approve of marriage between blacks and whites.

That last issue has undergone one of the greatest transformations in polling over the past 50 years. In 1960, just 4 percent of Americans approved.

More than a triumph over private prejudice, these numbers reflect changing social norms. The country hasn’t extinguished racism. But society — universities, employers, cultural institutions, the military — has made clearer over time that people who hold racist views had better nurse them off in the corner.

But these norms may be fraying. Since the last presidential election, and particularly since white supremacists rallied this month, unmasked, in Charlottesville, Virginia, the line between acceptable and ostracized views has started to become less stark. When President Donald Trump declined to condemn white supremacists more forcefully, he ignited a fight that at its core is about how we define norms in America: Who gets to be part of civil society, and whose views belong on the fringe?

That fight is being waged by opposing protesters across the country and by pundits daily on TV. The president’s critics fear that he is inviting white supremacists out of the corner, helping ideas that have become widely reviled in America to be redefined as reasonable opinions — just part of the discussion.

“They are explicitly trying to do that,” Tina Fetner, a sociologist at McMaster University in Ontario, said of members of white supremacist groups.

Until recently, they were ignored. But now the president is repeating their memes and the distorted versions of history that prop up their views, she said. As a result, the media is broadly covering them, too.

“This is exactly the process of how social change happens,” Fetner said. “It’s not because all of a sudden there is more racism now than there was a few weeks ago. It’s that the absolute condemnation of those most abhorrent views is crumbling away because the president isn’t fulfilling that role.”

Fetner has studied the transformation in views on gay rights and same-sex marriage. The shifts around race and gender similarly reflect not just widening acceptance of equality but also the rising condemnation of anyone who vocally opposes it.

Polls don’t necessarily capture how people truly feel; they capture what people are willing to say to a pollster. But the idea that some people might lie in surveys illustrates how social norms work. And political scientists suspect that part of what Trump has done, through his anti-immigrant and nativist appeals, is encourage people who might have kept silent in the past about their racist views to express them in public.

“For all these years, this is a group of people that’s been very bitter about the fact that they feel like they can’t speak,” said Sarah Sobieraj, a sociologist at Tufts University. “It’s not just that their policies haven’t been popular.”

And then Trump says similar things, with a powerful platform, without apology.

When norms of acceptable behavior and speech start to shift, it can disturb the shared beliefs, values and symbols that make up our culture.

“It’s really all of those things that we’re watching right now — they’re all up for discussion,” Sobieraj said.

Far-right activists gathering in Boston over the weekend were outnumbered by thousands of counter-protesters seeking to show whose ideas constitute the fringe. Employers and community members have communicated the same over the past week, shaming and firing some of the men rallying in Charlottesville who’ve since been publicly identified.

As the president has equivocated on their views — some are “very fine people,” he said last week — less powerful but more numerous voices are trying to make clear that they disagree.

Social scientists know that political leaders and institutions play an essential role in establishing norms. The American Psychological Association began to shift people’s views in the 1970s when the group declared that homosexuality is not a mental illness. The U.S. military did the same with racial equality when it began to desegregate in the late 1940s.

Tali Mendelberg, a Princeton University political scientist who has written about norms around racial equality, points to several forces that nudged the changing views. The urbanization of the country exposed more people to egalitarian cultural ideas, and the nonviolent tactics of the civil rights movement exposed Americans to the brutality of racism. Big wars, she added, have helped advance racial equality.

“When the country needs African-Americans, to fight in the trenches or power the factories, it tends to make more room for them in the American mosaic,” Mendelberg wrote in an email.

When norms change, the highly educated tend to adopt them the fastest. And when political leaders agree, those attitudes spread through the population the more information people have about them. When political leaders don’t agree, attitudes tend to polarize (for example, liberals say climate change is human-driven; many conservatives say that it’s not).

Polarized issues have two-sided information flows, as John Zaller, a political scientist at the University of California, Los Angeles, has described it. Trump’s comments about Charlottesville raise the possibility of creating a two-sided issue out of racial equality.

”That’s what’s really dangerous about what’s happening right now,” said Michael Tesler, a political scientist at the University of California, Irvine. “There should be a one-sided information flow condemning Nazis. And when there’s not, it’s very problematic.”

We are in a very different moment from the early 2000s, when Sen. Trent Lott, a Republican, praised Strom Thurmond in a way that critics said was an endorsement of segregation. Members of both parties condemned Lott, for comments that were no less offensive to many than some of Trump’s today. Lott was forced to step down as Senate majority leader. A crucial norm — politicians don’t celebrate racial segregation — asserted itself.

“And who’s going to do that to Donald Trump? His voters aren’t, because a lot of them like what he’s saying,” Tesler said.

Republican Party leaders appear constrained, too, he said. “How are you going to sanction Donald Trump?”