Steven Hahn is a professor of history at the University of Pennsylvania and the author of “A Nation Under Our Feet: Black Political Struggles in the Rural South from Slavery to the Great Migration.”

THE white students at Ole Miss who greeted President Obama’s decisive re-election with racial slurs and nasty disruptions on Tuesday night show that the long shadows of race still hang eerily over us. Four years ago, when Mr. Obama became our first African-American president by putting together an impressive coalition of white, black and Latino voters, it might have appeared otherwise. Some observers even insisted that we had entered a “post-racial” era.

But while that cross-racial and ethnic coalition figured significantly in Mr. Obama’s re-election last week, it has frayed over time — and may in fact have been weaker than we imagined to begin with. For close to the surface lies a political racism that harks back 150 years to the time of Reconstruction, when African-Americans won citizenship rights. Black men also won the right to vote and contested for power where they had previously been enslaved.

How is this so? The “birther” challenge, which galvanized so many Republican voters, expresses a deep unease with black claims to political inclusion and leadership that can be traced as far back as the 1860s. Then, white Southerners (and a fair share of white Northerners) questioned the legitimacy of black suffrage, viciously lampooned the behavior of new black officeholders and mobilized to murder and drive off local black leaders....