Strangers stared at them on the street. The motel front-desk clerk couldn't grasp they were a couple. A passerby mistook their child as adopted.

The relationship between Bill de Blasio, who is white, and Chirlane McCray, who is black, was once a spectacle even on the multiethnic streets of New York City. Now, that relationship has helped propel Mr. de Blasio into City Hall, and put the couple and their two children among the nation's most prominent interracial families, a symbol of changing attitudes.

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