LOS ANGELES — Carrying protest signs, thousands of teachers and their allies converged last month on the shimmering contemporary art museum in the heart of downtown Los Angeles. Clad in red, they denounced “billionaire privatizers” and the museum’s patron, Eli Broad. The march was a preview of the attacks the union would unleash during the teachers’ strike, which ended last week.

As one of the biggest backers of charter schools, Mr. Broad helped make them a fashionable and potent cause in Los Angeles, drawing support from business leaders like Reed Hastings, the co-founder of Netflix; Hollywood executives; and lawmakers to create a wide network of more than 220 schools.

Mr. Broad was so bullish about the future of charter schools just a few years ago that he even floated a plan to move roughly half of Los Angeles schoolchildren — more than 250,000 students — into such schools. In 2017, he funneled millions of dollars to successfully elect candidates for the Board of Education who would back charters, an alternative to traditional public schools that are publicly funded but privately run.

His prominence has also turned him into a villain in the eyes of the teachers’ union. Now Mr. Broad and supporters like him are back on their heels in Los Angeles and across the country. The strike is the latest setback for the charter school movement, which once drew the endorsement of prominent Democrats and Republicans alike. But partly in reaction to the Trump administration, vocal Democratic support for charters has waned as the party has shifted further to the left and is more likely to deplore such schools as a drain on traditional public schools.