PHILADELPHIA — In his marathon primary fight against Hillary Clinton, it became pretty obvious that Sen. Bernie Sanders had a tough time reaching out to black voters. It was a fairly simple but seemingly stubborn riddle he never quite solved – despite having an African-American communications director and forceful black surrogates in Ohio State Sen. Nina Turner and Ben Jealous, former NAACP president.

Whether the Vermont senator or his supporters are still interested in solving it, however, is another matter.

In the opening session of the Democratic National Convention Monday, during a powerful speech by Rep. Elijah Cummings of Maryland – civil rights champion, Congressional Black Caucus heavyweight and son of a sharecropper – which largely focused on the Democrats' fight for civil rights, Sanders supporters began chanting, "No TPP!" and holding ups signs, essentially heckling Cummings loudly on national TV.

Cummings was co-chair of the Democrats' platform committee, which inserted language that didn't specifically oppose the Trans-Pacific Partnership, a sweeping trade deal that President Barack Obama put high on his second-term wish list. But the deal, and the language in the platform essentially protecting it, is a flashpoint for Sanders' backers; they believe trade pacts are responsible for triggering the decline of American manufacturing and causing wages to stagnate.

Sanders used Clinton's support for TPP as a cudgel against her in the primary debates. But his supporters publicly harassing Cummings, a widely respected lawmaker and African American leader, isn't a good look for a faction that, by Sanders' own admission, got clobbered by Clinton in the South, home to the largest segment of the nation's black population.