Organizers are also reaching out by email and old-fashioned phone banks.
"The general rule is you keep people on the platform they're used to," Parcher said. "If they're on Facebook, we'll ask them to post something to Congress members' pages."
Gabriel Villalobos, a Spanish-language talk radio host in Phoenix, said many of his callers believe it is the wrong time for marches, fearful that that any unrest could sour public opinion on immigration reform. Those callers advocate instead for a low-key approach of calling members of Congress.
"The mood is much calmer," said Villalobos, who thinks the marches are still an important show of political force.
Salas, whose group is known as CHIRLA, dates the May Day rallies to a labor dispute with a restaurant in the city's Koreatown neighborhood that drew several hundred demonstrators in 2000. Crowds grew each year until the House of Representatives passed a tough bill against illegal immigration, sparking a wave of enormous, angry protests from coast to coast in 2006.
The rallies, which coincide with Labor Day in many countries outside the U.S., often have big showings from labor leaders and elected officials.
Aside from Los Angeles, big crowds were expected in New York, Chicago and Milwaukee. At a rally in Salem, Ore., Gov. John Kitzhaber planned to sign legislation to authorize drivers' licenses to people in the state illegally. With Congress in recess, there were no major demonstrations planned in the nation's capital.
Organizers were sending text-message blasts on Tuesday to remind subscribers of times and places for the marches.