Monday marked Martin Luther King Jr. Day, a national holiday honoring a man best remembered for urging Americans to judge one another by the content of their character rather than the color of their skin. It is a legacy rooted in a specific historical struggle -- one that culminated in the Civil Rights Act after years of fierce resistance, largely from Southern Democrats, to dismantling Jim Crow.

Since that era ended, the Democratic Party has repeatedly searched for what might come next: a successor to the civil rights movement that once defined its moral authority. Over the decades, a series of causes have been framed in those terms, often with strained comparisons to the racial discrimination of the mid-20th century.

First came a movement centered on gender, casting women as victims of a patriarchal system in a way likened to black Americans' experience under segregation. The analogy never quite fit. Later, similar language was applied to issues of sexual orientation and........

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