The civil rights movement knocked down voting restrictions, segregated lunch counters, employment discrimination and whites-only public schools. But fair housing has remained a far more elusive front for the last half-century. American communities are still deeply segregated by race. And segregation that has long divided U.S. cities is now emerging in the suburbs.

Equal access to housing has proven such an intractable goal because of the way past discrimination is built into the environment — leaving residential patterns and sprawling housing projects long after the decisions that created them — and because the forces that reinforce racial segregation today have grown much harder to see.

Now civil rights groups and housing advocates are deeply worried they may lose what they consider their strongest tool for dismantling segregation in a case the Supreme Court is scheduled to hear today. The case, Texas Department of Housing and Community Affairs v. The Inclusive Communities Project, poses a fundamental question about the power of the 1968 Fair Housing Act, which outlawed discrimination on the basis of race in housing decisions:

Does the law prohibit only intentional discrimination, or does it also apply to seemingly race-neutral policies that still have the effect of harming minorities?

The first type of discrimination — landlords refusing to rent to blacks, or housing agencies building projects only in black ghettos — has grown rare. It's the second form of harm, housing advocates argue, that remains common. They see it in zoning laws that effectively block the construction of housing for low-income families; in banking practices that disproportionately saddled qualified minority borrowers with subprime loans; and in subtle real estate conventions that steer blacks away from white neighborhoods.

"Most racially integrated places don’t stay integrated, and the reason they don’t is because of steering, because of exclusionary zoning, because mortgage lending doesn’t give black families the same ability to leverage their assets as white families," says Myron Orfield, a law professor and the director of the Institute on Metropolitan Opportunity at the University of Minnesota. "This is really the only tool to get at a lot of this discrimination."

The Texas case now before the court, which dates to a 2008 lawsuit, challenged the system the state housing agency used to allocate low-income housing tax credits to developers for the construction of housing affordable to poorer families. The Inclusive Communities Project, a group that advocates for more integration in the Dallas suburbs, argued that the state allocated these tax credits such that new low-income housing in the area was built almost exclusively in high-poverty minority neighborhoods. The system, the group argued, denied minorities access to affordable housing in communities where they might access better schools and greater opportunity.

State officials were not charged with intentionally excluding blacks from white neighborhoods. But the result of the system assigning tax credits effectively did that.

For decades, the idea that the Fair Housing Act bars housing policies and practices like this one that, to use the widely cited legal term, have a "disparate impact" on minorities has not been particularly controversial. Eleven circuit courts have interpreted the law this way. And the Department of Housing and Urban Development, charged with enforcing the Fair Housing Act, issued a rule in 2013 explicitly interpreting the law to cover "disparate impact" claims.

Under Chief Justice John G. Roberts Jr., however, the Supreme Court has repeatedly sought such a case to take up the question — even though there had been no disagreement among lower appeals courts. Indeed, the court has tried to take up two "disparate impact" fair housing cases before, but both settled before the court had a chance to weigh in, to the relief of civil rights groups worried about a decision that could gut the law.

Civil rights groups, which have filed amicus briefs en masse in this case, are wary that this increasingly conservative court, which two years ago dismantled a key provision of the Voting Rights Act, would do the same with another foundational piece of civil-rights legislation from that era.

"This was really the last legislative victory of the civil rights movement, and it was Dr. King’s last victory, too," says Philip Tegeler, the executive director of the Poverty & Race Research Action Council. Congress passed the Fair Housing Act in a bipartisan vote a week after King's assassination in 1968, in a nod to one of his final campaigns. "This is the message that Dr. King brought to Chicago in 1966, talking about de facto segregation in the north, segregation wherever it exists — that we need to address it. That’s in large part what the Fair Housing Act was trying to do."

King believed that housing desegregation was not merely a matter of ensuring decent living conditions for blacks, but of creating opportunity as well. As remains true today, housing influences where children go to school, whether they live in healthy environments, what kinds of jobs their parents have access to, and how families amass wealth over time.

As the Supreme Court weighs the matter, civil rights groups and the 17 states and many cities siding with them argue that there should be no debate over how Congress originally intended the law to be used. Some of the Fair Housing Act's original co-sponsors have signed on to an amicus brief siding with the Inclusive Communities Project, including Edward Brooke, the first popularly elected black senator in America who died earlier this month. George Romney, head of the Department of Housing and Urban Development at the time, was himself a major supporter of policies designed not just to dismantle segregation, but also to foster more integrated communities.

"It was George Romney, Richard Nixon and Warren Burger who created 'disparate impact' for fair housing," Orfield says. "It wasn’t a liberal idea, it was a very middle-of-the-road, conservative idea."

The housing industry counters that it's more costly to build new affordable housing in wealthier neighborhoods, and that the fear of disparate impact lawsuits might perversely discourage developers from taking on affordable-housing projects. Critics also argue that disparate impact claims unfairly impugn the motives of whites, associating neutral policies with discrimination when none was intended.

Tegeler, though, argues just the opposite, that disparate impact claims allow the government to address policies that effectively harm minorities without accusing anyone of overt discrimination.

"You don’t have to charge someone with being personally motivated by bias," he says. "I think we lose a lot if we lose the capacity to have that conversation."

Without "disparate impact," Orfield argues that it would be much harder to go after banks that offer better mortgage terms to white families making $40,000 a year than black families who earn three times as much. It would be much harder to target exclusionary zoning policies that prohibit, for instance, the construction of multi-family buildings or smaller homes that offer affordable housing to families. These results are grounded in seemingly neutral financial formulas, not overt racial animus.

"You don’t have public officials in these communities saying, ‘We don’t want any blacks here,’ " Orfield says. "They’re saying, ‘We’re not going to build apartment buildings because they don’t have a very good fiscal dividend.’ "

Without "disparate impact," fair housing advocates argue that they will be left trying to prove the impossible.

"No government official and no developer is going to say that 'the reason I’m doing this is because I'm a racist and I want to lock up black people in segregated neighborhoods,' " says Richard Rothstein, a research associate with the Economic Policy Institute. "If that becomes the standard, then we have given up on racial integration in society."