The union representing Los Angeles teachers has pulled together a coalition of other employee unions to oppose a controversial plan to more than double the number of local students attending charter schools.

The linking of the unions is a potentially significant step in the showdown over charter expansion, which, if successful, would alter the education landscape in the city.

The proposed $490-million effort, spearheaded by the Eli and Edythe Broad Foundation, would enroll half of L.A. Unified students in charter schools over the next eight years.