They’re making sure communities of color have a voice by organizing caucus trainings and setting up dedicated Spanish-language voting locations. On a Saturday night in January, several dozen men stepped out into the single-digit cold and ventured to East High School for a soccer tournament.

But before they began playing, campaign staffers for Sen. Bernie Sanders (I-Vt.) taught them how to participate in the Iowa caucuses on Feb. 3.

Luis Gomez, 31, is the Sanders die-hard who organized the event. He was never involved in politics until he...