The union that has struggled for years to get a foothold at one of the Japanese-owned auto assembly plants in Ontario is poised for a potential breakthrough.

Skilled-trades workers at Toyota Motor Manufacturing Canada Inc. plants will vote on Friday and Monday on whether to join Unifor, the successor to the Canadian Auto Workers (CAW), which has knocked on the door at Toyota for the better part of two decades, but has never won a union vote.

Unifor has signed up more than 50 per cent of the approximately 500 electricians, millwrights and other skilled workers at plants in Cambridge, Ont., and Woodstock, Ont., said John Aman, the union's director of organizing, on Thursday. A vote of 50 per cent plus one would mean the union would be certified as the representative for those maintenance workers and then negotiate a contract.

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However, the key test will be a decision by the Ontario Labour Relations Board on whether a bargaining unit of that size can be separated out from the entire work force at the two plants, which numbers about 8,000. "The only dispute is whether it's an appropriate unit that we're seeking to represent," Mr. Aman said.

Unifor's most recent attempt to win certification at the two plants was withdrawn in 2014. At the time, the union thought that enough workers had signed union cards to enable it to win a certification vote. But when the company revealed the actual number of employees at the plants, the union realized that it had not reached the 50-per-cent threshold.

The attempt to organize skilled-trades workers does not affect the union's attempt to organize the entire work force, Mr. Aman said. "We always want to organize a bigger group and that will still be our goal in the long term, but as of right now, the support we have is with this [smaller] group," he said.

While Unifor and the CAW tried to organize workers at Toyota and Honda of Canada Manufacturing in Alliston, Ont., after the Japan-based companies began making vehicles in Canada, the union's membership at the Canadian units of the Detroit Three auto makers declined steadily.

Chrysler (now Fiat Chrysler Automobiles NV), Ford Motor Co. and General Motors Co. closed assembly plants and parts facilities in Ontario, eliminating more than 10,000 jobs.

Toyota Motor Canada said in a statement that it does not believe a union has "anything to offer to our workplace."

The company said the application for certification is designed to separate maintenance and production workers, which could eventually affect all workers. "Production and maintenance teams, in particular, are well integrated in our organization and their teamwork is a necessity each and every day," the statement said.