Last week in Iowa, a host at a minority-focused Brown and Black Forum asked Mr. Sanders if he would be in favor of “reparations for slavery.”

“No, I don’t think so,” Mr. Sanders said. “First of all, its likelihood of getting through Congress is nil. Second of all, I think it would be very divisive.”

Mr. Coates wrote that Mr. Sanders’s opposition to reparations because it was not likely to get through Congress didn’t make sense because many of the policies the senator is proposing — such as tuition-free public colleges, single-payer Medicare for all, and a $1-trillion jobs and infrastructure bill — was also not likely to be passed.

“Sanders says the chance of getting reparations through Congress is ‘nil,’ a correct observation which could just as well apply to much of the Vermont senator’s own platform,” Mr. Coates wrote. “If this is the candidate of the radical left — then expect white supremacy in America to endure well beyond our lifetimes and lifetimes of our children. Reparations is not one possible tool against white supremacy. It is the indispensable tool against white supremacy. One cannot propose to plunder a people, incur a moral and monetary debt, propose to never pay it back, and then claim to be seriously engaging in the fight against white supremacy.”