On a few occasions, concerns that illegal immigrant labor would undermine UFW strike campaigns led to a number of controversial events, which the UFW describes as anti-strikebreaking events, but which have also been interpreted as being anti-immigrant. In 1973, Chavez and members of the UFW marched through the Imperial and Coachella Valleys to the border of Mexico to protest growers' use of illegal immigrants as strikebreakers. Joining him on the march were Reverend Ralph Abernathy and U.S. Senator Walter Mondale. In its early years, the UFW and Chavez went so far as to report illegal immigrants who served as strikebreaking replacement workers (as well as those who refused to unionize) to the Immigration and Naturalization Service.