They were the pink slips that helped change American liberalism.

Forty years ago — on May 9, 1968 — several unionized white educators received word that they had been summarily fired by the local school board of Ocean Hill-Brownsville, a black neighborhood in Brooklyn.

Liberals in New York were not sure how to react. When white people fired black people for no cause, liberals knew it was wrong; when conservative employers arbitrarily fired unionized employees, they knew which side they were on. But what was one to think when black people were firing white people, and when the assault on labor unions came from the left?

The controversy unleashed a civil war within American liberalism, tearing apart groups that had hitherto been allies: black people and Jews, and civil-rights groups and organized labor.

Most upper-middle class liberal New Yorkers, including the leadership of the New York Civil Liberties Union and the editorial pages of the New York Times, were sympathetic to the black community school board. The Ocean Hill-Brownsville board had been established as part of an effort to give poor minority communities greater say over the affairs of New York City schools.

Part of the idea behind community control was that students of color would perform better if local school boards hired more minority teachers as role models, using race as a factor in decisions. Community control was backed by an unlikely coalition of “black power” activists, like Sonny Carson, and white, patrician liberals, like Mayor Lindsay and the president of the Ford Foundation, McGeorge Bundy.

But a second camp of liberals, led by the 39-year-old head of the United Federation of Teachers, Albert Shanker, took a different view. Shanker, the son of a newspaper deliverer and a seamstress, was a strong advocate of civil rights. He had traveled with a contingent of teachers to hear Martin Luther King Jr.’s address at the 1963 March on Washington, and had marched with King in Selma in 1965.

While Shanker understood and sympathized with the need for more black teachers, he thought firing (or hiring) based on race was antithetical to what the civil-rights movement had been about. He believed that King’s universal message — that people be judged not “by the color of their skin but by the content of their character” — was fundamental to the moral power of the movement, not something to be casually dismissed, as Bundy seemed to suggest.

And while Shanker agreed with Bundy that one shouldn’t just maintain the status quo of racial exclusion and the legacy of segregation, he argued for taking affirmative action that helped the economically disadvantaged of all races, an approach that King embraced as well.

King proposed a Bill of Rights for the disadvantaged, not a Bill of Rights for blacks, saying: “It is a simple matter of justice that America, in dealing creatively with the task of raising the Negro from backwardness, should also be rescuing a large stratum of the forgotten white poor.”

The Ocean Hill-Brownsville board, however, had a very different vision in mind. Its superintendent, Rhody McCoy, took his inspiration not from King but from Malcolm X, whose home he had visited on many occasions. McCoy made clear that his ultimate goal was an all-black teaching force in his district.

In response to the firings, Shanker and UFT members voted to go on what would turn out to be a series of three strikes, between September and November in 1968, throwing 1 million students out of school for a total of 36 days. At the time, it was the largest and longest set of school strikes in American history.

Ultimately, the public was on Shanker’s side, and the Ocean Hill-Brownsville experiment was shut down. Although Shanker was victorious, he was despised by the chattering class. Woody Allen, a good gauge of New York liberalism, made Shanker the butt of a joke a few years later in his science-fiction comedy, “Sleeper.” In the film, Mr. Allen’s character wakes up 200 years in the future to discover that civilization was destroyed when “a man by the name of Albert Shanker got hold of a nuclear weapon.”

In the real world, however, Shanker immediately set out to repair the labor-civil-rights alliance, which lay in tatters, by finding a way to increase the number of black teachers in New York without resorting to racial hiring or firing.

In 1969 he sought to unionize the city’s teachers’ aides, known as paraprofessionals, who were mostly poorly educated black and Hispanic welfare mothers. Shanker wanted to improve their wages and negotiated a career ladder for them, including a stipend so they could go back to school, earn high school diplomas and college degrees, and become full-fledged teachers. By the time of Shanker’s death in 1997, the career-ladders program had helped more than 8,000 paraprofessionals to become teachers, making the program the largest source of minority teachers in New York City.

Civil-rights groups and Democrats, however, took a different line, endorsing not the difficult and costly task of providing advancement programs for low-income and working-class people of all races, but rather championing a race-specific program of preferences. It was a pivotal decision, and a turning point for American liberalism.

Forty years after Ocean Hill-Brownsville, however, liberals have a unique opportunity to heal old wounds. Barack Obama’s candidacy offers the possibility of resolving the difficult question raised in Ocean Hill-Brownsville: how to remedy the history of discrimination in this country without creating new inequities and divisions?

Senator Obama, who sounds far more like King than McCoy, has said that his own relatively privileged girls don’t deserve affirmative-action preferences, but poor minority and white students do. Such a change would be transformative, recapturing not only the colorblind character of King’s vision but also its aggressive assault on class inequality. And it would, at long last, turn the page on the divisiveness of Ocean Hill-Brownsville.

Mr. Kahlenberg, a senior fellow at the Century Foundation, is the author of “Tough Liberal: Albert Shanker and the Battles Over Schools, Unions, Race, and Democracy.” This article was adapted from a longer piece that appeared in the Chronicle of Higher Education.