Few Supreme Court opinions have resonated through the ages more than John Marshall Harlan’s lone dissent in Plessy v. Ferguson, the 1896 case establishing the infamous “separate but equal” doctrine. “There is in this country no superior, dominant, ruling class,” Harlan proclaimed. “Our Constitution is color-blind,” he continued, “and neither knows nor tolerates classes among citizens.” Outnumbered seven to one, Harlan expected that the ruling would tarnish the court’s reputation and poison racial relations for generations.

In his wide-ranging biography, “The Great Dissenter: The Story of John Marshall Harlan, America’s Judicial Hero,” Peter S. Canellos, executive editor at Politico, convincingly argues that Harlan was the brightest star on the Supreme Court during the industrial era and a visionary for his outspoken and prescient dissents in the iconic civil rights and economic cases of his time — none more so than Plessy. But Canellos also invites us to consider the less-heroic aspects of Harlan’s career, most of all his unwillingness to vigorously defend the rights of Asian Americans.

Those who know Harlan only because of his most famous dissent will be surprised by the debate surrounding the justice’s evolving and complicated legacy. During his lifetime, liberals praised Harlan’s views on economic regulation, and African Americans revered his support of civil rights. These encomiums stood in stark contrast to Harlan’s reputation among many of his contemporaries and later generations of legal chroniclers. Unable to appreciate his prophetic rulings, they dismissed him as an eccentric. His stature rose only after Thurgood Marshall christened his dissent in Plessy as the “bible” of the NAACP’s legal crusade combating segregation. Still predominant, this viewpoint, which Canellos also holds, has been clouded by an unsettling question in recent years: Did Harlan, a champion of African American equality, condone discrimination targeting other minorities? Would he have endorsed affirmative action, as liberals claim? Or would he have opposed such efforts because they treat racial groups differently, as conservatives, including Justice Clarence Thomas, now assert?

Though Canellos addresses these questions, they remain unresolved. Instead of the definitive say on Harlan’s legacy, this biography offers thoughtful contributions to these ongoing debates and insights into the principles shaping the justice’s thinking, bringing us closer to comprehending his unsettled legacy and, more tellingly, casting a light on why it has morphed over time.

Harlan was an anomaly among his peers on the Supreme Court. Often plucked from the coterie of wealthy corporate attorneys representing the robber barons, his brethren perceived attempts to mitigate the scourges of the Gilded Age — child labor, hazardous workplaces and monopolies — as threats to their faith in laissez-faire capitalism and social Darwinism. Far more progressive than his colleagues, Harlan envisaged a robust federal government capable of addressing the nation’s economic ills and challenging corporate power. As a man of modest means, he was also sensitive to the consequences of income inequality and the imbalance of power between employers and workers. Although Harlan spent his years on the court, 1877 to 1911, relegated to the losing side of these cases, many of his dissents eventually became the law of the land.

The Supreme Court’s civil rights record during Harlan’s tenure was far more appalling than its recalcitrant approach to corporate reform. In a series of cases culminating in Plessy, the court gutted the constitutional protections afforded to African Americans during Reconstruction. In doing so, it granted its imprimatur to the South’s apartheid regime. Operating with carte blanche, Southern governments extended Jim Crow to all facets of life. Nothing symbolized the unbridled white supremacy engulfing the region more than the thousands of lynchings across the South.

The mystery surrounding Harlan is why he, a former Kentucky enslaver who had opposed abolition and Abraham Lincoln’s reelection, came to support racial equality. Canellos provides a credible explanation. To start with, Harlan’s devout religious views and sense of duty sacrificed individual and sectional aspirations for the greater good. During the Civil War, that meant holding Kentucky — a slave state — in the Union and serving in battle. After the war, it meant keeping Reconstruction alive in the face of widespread indifference to the plight of African Americans.

Harlan’s close relationship with Robert Harlan, a biracial, formerly enslaved man whom some at the time suspected was the justice’s half brother, may have played the biggest role. Though his race afforded him limited education and professional prospects, the enterprising polymath earned a fortune during the California gold rush and became a prominent intellectual, a treasury official and a member of the Ohio legislature. An “aristocrat of color,” Robert Harlan was among the most influential and successful Black men in America. Canellos persuasively argues that witnessing Robert Harlan — the “living refutation of Black inferiority” — succeed in the face of herculean obstacles inspired the justice’s enlightened views.

The primary threat to Justice Harlan’s sterling legacy is his lack of solicitude for Asians plagued by discrimination. In a pair of cases, Harlan supported attempts to impede Chinese immigration and block American-born Chinese from becoming citizens. In Harlan’s defense, Canellos contends that the justice’s decisions arose not from racial animus but from his long-held belief in robust federal authority over immigration policy and treaty-making. But, more disturbingly, they also emerged out of his “deep-rooted skepticism about whether people who lived under monarchies could adapt to American democracy.”

Canellos adds that Harlan applied his stance on assimilation equally to Asians and Europeans. When assessing these cases through Harlan’s views of federal power, Canellos’s argument has merit. Plus, the justice’s record involving constitutional protections for other minorities was more progressive.

Still, to modern eyes, the optics are disconcerting: Harlan’s thunderous call for Black equality was, at times, notably absent in cases involving other marginalized groups. Ironically, this very dichotomy appeared three paragraphs after Plessy’s famed “color-blind” passage. “There is a race so different from our own that we do not permit those belonging to it to become citizens,” Harlan wrote, voicing no consternation about the exclusion of the “Chinese race” from the fabric of American life even as he cited the example to condemn Black segregation.

Where does this leave Harlan’s legacy? On a Supreme Court widely decried for handicapping government reforms during industrialization, his jurisprudence in this arena was farsighted. As the most powerful American to champion Black equality after Reconstruction, he was a beacon of light in a time of darkness. Yet his failure to universally apply the Constitution’s protections besmirches his record. Under these circumstances, should Harlan be revered or reviled? Canellos does not definitively settle this question, but the ways the rest of us attempt to answer it will surely influence contemporary constitutional debates and reveal a great deal about our own changing values.

The Great Dissenter

The Story of John Marshall Harlan, America’s Judicial Hero

Peter S. Canellos

Simon & Schuster

624 pp. $32.50