Forty-eight years ago Thursday, five hundred or so activists gathered to march from Selma, Alabama, to Montgomery to protest the denial of voting rights to blacks in the state. They didn’t make it. The march was attacked by state and local police, who were cheered on by crowds of white onlookers in an assault so brutal that it has come to be known as Bloody Sunday. Seventeen people, including future congressman John Lewis, were hospitalized. Last weekend, an array of activists and elected officials gathered in Selma, as they have for many years, to commemorate that march. But this year, the commemoration had a special significance: it came just days after the Supreme Court heard arguments in Shelby v. Holder, a case that threatens to eviscerate the Voting Rights Act, which might never have passed were it not for the aborted Bloody Sunday march and the chaotic, violent tableau playing out in Alabama. If we take nothing else from this anniversary, it’s a reminder that the history of race in this country resembles a pendulum, not an arrow.

The debates over Shelby v. Holder are not solely about the future of the Act but about the claims of the past on the present, and the fragile nature of progress. This nation’s racial history has been plagued by a law of unintended consequences; moments of apparent progress have opened the doors to regression. The possible gutting of the Voting Rights Act in the midst of a black Presidency isn’t contradictory—it’s consistent with the long-term trend. In 2007, as Barack Obama’s campaign gained momentum, a friend who’d worked on Jesse Jackson’s 1988 bid said to me, “If that guy wins, the V.R.A. is out of here.” He wasn’t being cynical so much as historical—and, if the consensus on the Court’s perspective on Shelby holds true, prescient.

There is no way to excuse the three-fifths clause of the Constitution—you can’t even rightly claim it as some kind of example of honorable compromise, as the president of Emory University, James Wagner, tried to do in a column in his school’s magazine recently, unless your definition of a successful compromise extends to a civil war and six hundred thousand dead—but the successful efforts to excise that clause from the document yielded conditions that were in some ways even worse than those that had prevailed under it. The Fourteenth Amendment established that the recently-emancipated slaves were citizens, and that in place of the old system of counting only sixty per cent of the slave population, representation would be determined by “counting the whole number of persons in each state.” In political terms, the Amendment effectively increased the population of the South by 1.6 million people. On its heels came the Fifteenth Amendment, which secured the right of black men to vote. The amendments, in conjunction with the election of hundreds of blacks to political office during Reconstruction, filled some observers with the hope that the nation was well on its way to reconciling its practices with its ideals, becoming what we would now call “post-racial.” But Southerners effectively nullified the Fifteenth Amendment through concentrated, unrestrained violence targeted at black voters, and the provision in the Fourteenth Amendment that was intended to punish that kind of behavior, by reducing the representation of any state that denied those eligible to vote the right to do so, was ignored.

The fractured bones and lacerated flesh of the marchers in Selma were not simply a testament to the sclerotic souls of the bigoted South; they were a measure of Southern commitment to the principle of one-man, one-and-a-half votes. Bloody Sunday and the bloodletting days that preceded it were about retaining the political advantage the South had gained under the three-fifths clause and had seen expand after the Fourteenth Amendment.

In 2007, Barack Obama traveled to Selma to address a civil-rights establishment that was somewhat skeptical of his embryonic Presidential campaign. Obama used the occasion to make common ground with the movement, to point to the kindred deprecations of colonialism in Kenya and Jim Crow in Alabama. He took pains to remind the audience that, without the civil-rights movement, his parents’ marriage would not have even been recognized by the state of Alabama. And it was surely not lost on Obama—or anyone in the audience—that the Voting Rights Act would be crucial to any electoral path he planned to pursue in his White House bid.

By any reasonable measure, the Voting Rights Act is one of the most effective pieces of legislation of the twentieth century. At the time of its passage, just nineteen per cent of blacks in Alabama were registered to vote. Simple registration was a major accomplishment; actual voting was a life-threatening activity. Just five years later, more than sixty per cent of blacks in the state were on the rolls. By 1965, there were roughly a thousand black elected officials nationally—by 2000, there were nearly ten times that number, and of the ten states that showed the largest increase, eight had once seceded from the Union. And in 2008, and again in 2012, an African-American Presidential candidate won Virginia, which once held the capital of the Confederacy.

This does not mean that the problem has been solved, however. At the center of the current debate over the V.R.A. is its Section 5, which requires counties with a history of black-voter disfranchisement to submit any proposed changes to its electoral laws or districts to a federal panel for approval, or “preclearance,” to ensure there is no discriminatory impact. As the N.A.A.C.P. Legal Defense Fund brief in the case points out, the best evidence of the ongoing need for the V.R.A. is the data on preclearance itself. Texas, for instance, has been found in violation of the V.R.A. every time it has submitted its decennial redistricting plans. Louisiana has a similar track record.

Appreciating this situation requires subscribing to a particular kind of nuance. It is entirely possible to be both a country capable of electing a black President and one that still needs strong legal protections for black voters. The idea of the past as a pendulum is not an argument against the possibility of progress—it’s a warning against the belief that progress can easily and passively be made permanent. With every hours-long wait to get access to a voting booth, every voter-I.D. law, every gerrymandered district, and every attempt at voter suppression, history is close at hand. When he went before a joint session of Congress in March of 1965, a little more than a week after Bloody Sunday, to declare his support for legislation that would become the V.R.A., President Lyndon B. Johnson ended his speech with the words “we shall overcome.” Last weekend, marchers returned to Selma motivated in part by the fear that the Supreme Court believes we already have.

Photograph: AP