In a moving farewell speech before an enormous crowd in Chicago last week, President Obama evoked the American creed of unity and aspiration as the foundation of our democracy. He has always embraced a vision of America as a “melting pot.”

Mr. Obama embodied for many Americans the legacy of the Rev. Dr. Martin Luther King Jr., whom we celebrate on Monday. Our national memory of Dr. King has, for nearly 50 years, reinforced the belief that America, unlike any other nation, could extend opportunity to everyone regardless of his or her identity. In Dr. King’s name, assimilation and aspiration have been the keywords of the post-civil rights era, and diversity and inclusion its currency. And Mr. Obama has symbolized more than anyone in American history the idea that racial representation and the content of one’s character were the perfect antidote to racism.

It’s true that, in fulfilling the duties of the presidency with great dignity, Mr. Obama represents the highest expression of the goal of assimilation. But for African-Americans, he was also the ultimate lesson in how this antidote alone is insufficient to heal the gaping wounds of racial injustice in America. It’s clear that black leadership, in itself, isn’t enough to transform the country. So we must confront the end of an era and the dawn of a new one.

We now live in a post-assimilation America. The 50-year-old rules for racial advancement are obsolete. There is no racial barrier left to break. There is no office in the land to which an African-American can ascend — from mayor to attorney general and the presidency — that will serve as a magical platform for saving black people and our nation’s soul from its racist past. We cannot engineer a more equitable nation simply by dressing up institutions in more shades of brown. Instead, we must confront structural racism and the values of our institutions.