Cesar Chavez (not to be confused with, um, Hugo Chávez!) is probably best remembered as an incredible labor and civil rights leader. Along with the United Farm Workers--the union he helped found--he organized in innovative ways to lead a farmworker strike and grape boycott that brought California's agriculture kings to their knees. Chavez is also credited with popularizing the Spanish language phrase, "Sí, se puede," which can be translated to mean, "Yes, [we] can."
During a massive farmworker strike that first started in 1965, farmers sought to bring in undocumented laborers from Mexico. Those laborers, who were strikebreakers, were often called "illegals" and "wetbacks" by strike supporters--including by Cesar Chavez himself. Chavez himself was not an immigrant; his mother was brought to the United States as a newborn, and his father was born in Arizona. Most striking farmworkers were also Mexican-American, and the slurs could easily have offended those workers as well.
A copy of video of Chavez making these remarks on San Francisco public television station KQED in 1972 is now resurfacing, just a few weeks before the release of a major Chavez biopic staring Diego Luna and Rosario Dawson.