United Teachers Los Angeles President Alex Caputo-Pearl, left, joins teachers at a rally next to the Broad Museum in Los Angeles last month. Teachers in the nation's second-largest school district are set to strike Monday. (Damian Dovarganes/AP)

Tens of thousands of teachers in Los Angeles, the nation’s second-largest school district, are set to strike Monday after a months-long impasse with the school system, one of the largest acts of teacher activism in years.

The strike comes after more than 20 months of tense negotiations with the Los Angeles Unified School District. The parties continue to meet, but it appears unlikely they will reach an agreement before Monday.

It is the latest spate of teacher activism after a year in which educators in a half-dozen states walked out to protest work and classroom conditions. But this is the first such job action in a state controlled by Democrats.

United Teachers of Los Angeles, the union representing about 30,000 teachers, has demanded pay raises. But it also seeks smaller class sizes and more school nurses and counselors. The district said it cannot afford the union’s demands, and that agreeing to them would put the school system on the path to insolvency.

The district made plans to remain open even if teachers did not show up. In preparation, the school board has loosened requirements for parent volunteers and contracted with a company that could furnish substitute teachers en masse if the strike goes on, as first reported by the Los Angeles Times.

In a statement, the union urged parents and students to join them on the picket lines Monday.

“We know there are tough decisions ahead for the more than 600,000 students and their families impacted by a strike,” the union said in a statement. “While every family will make their own decision on whether to send their child to school in the event of a strike, having many parents and allies on picket lines will be powerful and transformative.”

Los Angeles Unified Board President Mónica García and Superintendent Austin Beutner on Wednesday traveled to Sacramento to ask state leaders for more resources in a last-minute attempt to prevent the strike. About 90 percent of the school system is funded by the state.

“We are working hard to avert a strike,” Beutner said in a statement. “We are building support at the state level to find more resources to help our students and better support all who work in our schools.”

The strike has crystallized the debate over the future of Los Angeles schools, where pro-charter school factions, backed by wealthy philanthropists, have been fighting the teachers union and advocates of traditional public schools. The enrollment in Los Angeles’s traditional system has been declining in part because students are leaving for charter schools. It has put a dent in the school system’s budget, because the state allocates education funding based on the number of students in a school. Charter schools are entitled to space in traditional public schools, an arrangement that has exacerbated tensions.

The union says the school system needs to invest if it wants to persuade families to return, part of why it has proposed reducing class sizes and bringing in more nurses and counselors. The district has said the financial picture is too bleak for those investments.