Washington Teachers’ Union members voted Monday evening to unseat their incumbent president in favor of a candidate who promised to more forcefully challenge school system management.

Veteran teacher and WTU activist Elizabeth Davis defeated Nathan Saunders with 55 percent of the vote in what both candidates said would be a game-changing election for the union, which is negotiating a new contract.

“It was a referendum on many fronts,” said Saunders, who received 380 votes to Davis’s 459. “They want more aggressive change than what I was dishing out.”

Davis’s running mate, Candi Peterson, was also victorious Monday in her bid to serve as the union’s general vice president, a position she held under Saunders until they had a falling out in 2011 and Peterson was forced out. Peterson, a social worker, writes a blog that has been fiercely critical of Schools Chancellor Kaya Henderson and her predecessor, Michelle A. Rhee.

It is not clear when Davis and Peterson will take over: They say immediately, citing union bylaws, but the WTU elections committee has said it won’t happen until Aug. 1.

In recent weeks, Saunders said he was close to finalizing a contract that would include salary increases and provisions that would allow for longer school days and a longer school year. Henderson supports those provisions.

Saunders said negotiations over that contract will fall to Davis, who said she would not comment on how she plans to proceed until she sees the pending contract language.

Davis said one of her first priorities will be to reverse Saunders’s agreement to change the terms of early retirement for teachers who lose their jobs because of budget cuts or school closures. That agreement with the school system, signed in December, shortchanges veteran teachers, Davis said.

“I hope that Chancellor Henderson will understand that the relationship with the union will have to change in some respects,” she said.

Henderson said in a statement that Saunders had been a “valued partner” and “great advocate for both teachers and students.” She offered congratulations to Davis and said she looked forward to working closely together.

Saunders was elected in 2010 after accusing then-WTU President George Parker of being too cozy with management. In office, Saunders sought to strike a cooperative relationship with Henderson, an approach he said was necessary to stay relevant and push for teachers’ interests at a time of nonunionized charter schools’ quick growth.

Davis, a longtime WTU activist, said Saunders ignored teachers who wanted a stronger voice pushing back against some of Henderson’s decisions, including her closure of 15 schools and her use of “reconstitution,” in which all teachers at a school must reapply for their jobs.

“We do not plan to be a roadblock to school reform or play to the stereotype of a union that blocks improvements, but we do not plan to be silent” on such issues, Davis said.

Davis added that teachers want more input in running the union and a stronger voice in shaping issues that affect teaching and learning, including curriculum, instruction and school climate.

“Teachers want the WTU to be less controlled by one person and more engaged with the full range of issues impacting teachers, students and schools,” she said. “We campaigned on a platform that said the union can be much better.”