I've posted before about sindicatos charros in Mexico being yellow dog unions.
But I ran into another Mexican use of the term here in the fabulous copy left Rini Templeton Art archive with this image, that came with the following description:
Over 5,000 workers at Mexico City's Universidad
Autónoma Metropolitana (SITUAM) struggled against wage ceilings imposed
by the government as part of an austerity program arranged with the
International Monetary Fund (IMF). They also advocated progressive
academic policies and solidarity with struggles for democracy. "Huelga
Huelga Huelga/Charro el que se raje"/Strike, strike, strike/Whoever
gives in is a sellout. Rini worked closely with them.