Dr. Martirano’s plan, which he called Equity in Action, would also alter the racial makeup of some schools, given that the majority of poor students in the county are black or Hispanic.

A growing body of research suggests that bringing students of disparate races and social classes together can boost children’s test scores and help them develop empathy. This year, all the leading Democratic presidential candidates have proposed desegregation strategies, a seismic shift after decades in which politicians from both parties played down the impact of the racial and class segregation that persists in American education.

Howard County is one of many school districts, from Dallas to New York City to San Francisco, now grappling with the challenges of integration. The virulent opposition in an area that its founder once declared to be “color blind” shows that the issue remains deeply divisive among liberals when it comes to their own children.

That there is a racial dynamic to the struggle in Columbia is undeniable. It is mostly white and Asian parents who are protesting the plan. Black and Hispanic children are more likely to be concentrated in schools with large numbers of poor students.

One piece of hate mail opposing the proposal said, “Blacks destroy school systems and schools.” Another said, “Certain families and communities do not have strong values in healthy family structure, high expectation on education, or firm beliefs in raising kids with good characters.”