A union representing Ontario's elementary school teachers has endorsed the New Democrats ahead of the spring election.

The Elementary Teacher's Federation of Ontario made the announcement at a meeting today.

ETFO president Sam Hammond says the party earned the endorsement because of its commitment to publicly funded education.

NDP Leader Andrea Horwath has promised to end standardized testing of students if elected and a cap on kindergarten class sizes, both proposals the teachers' union supports.

Teachers have had a strained relationship with the governing Liberals for years following a bitter contract dispute with former premier Dalton McGuinty in 2012.

The Liberals fired a volley at the NDP on the campaign trail after the ETFO's endorsement.

The Liberals issued a statement raising questions about NDP candidate Marco Coletta, who they suggest advocates for a 15 per cent reduction to teacher salaries in the province.

The tactic is one the Liberals have used repeatedly in recent weeks against candidates running for the PCs, including former Tory candidate and social conservative Tanya Granic-Allen, who was later turfed from the party roster.

Liberal Leader Kathleen Wynne says Coletta's views are reminiscent of the NDP's 2014 platform, in which she contends education was not a high priority.

She also said that the ETFO backing the NDP does not mean teachers themselves will vote that way.

"We haven't always had, as a party, the full support of all of the unions or the federations ... and individual teachers make their decisions on a riding by riding basis," she said.

Horwath says she isn't aware of the candidate's comments and said teachers would be "properly paid" for the work they do under an NDP government.

Numerous polls have shown building momentum for Horwath and the NDP.