Last year, Ken Coran led Ontario's secondary school teachers in a labour fight with the province's Liberal government and helped defeat the party in a crucial by-election. Now, he is set to be that government's candidate for a seat in the legislature, the Grits announced Friday.

The about-face in Mr. Coran's relationship with the Liberals is further indication Premier Kathleen Wynne has brought teachers back into her party's base of support, where they could play an important role in the next election.

In a statement, Mr. Coran credited Ms. Wynne with persuading him to run for the party in a by-election to fill the London West seat left vacant by the resignation of former energy minister Chris Bentley.

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"I believe in Premier Wynne, and it is her values of collaboration, openness and honesty that have drawn me into politics," he said.

Mr. Coran did not respond to interview requests.

The London by-election is one of five Ms. Wynne must call to replace Liberals who have resigned in recent months. The votes, which could happen as early as August 1, will be the party's first test of strength since she took over. Government sources said they have been told to expect to spend the summer in campaign mode.

Liberal insiders said the party recruited Mr. Coran, whose term as president of the Ontario Secondary School Teachers' Federation ends this month., and added that Mr. Coran did not actively seek the nomination before they approached him. Mr. Coran has been involved with the local riding association for years and its president, Scott Courtice, said both the association and party brass wanted him for the job.

The riding association is not recommending anyone else for the nomination, meaning Mr. Coran will get the nod unopposed at a meeting Tuesday.

When former premier Dalton McGuinty moved to impose contracts on teachers last year, the OSSTF fought back. The union campaigned against the Liberals in a by-election in Kitchener-Waterloo last September, where Mr. McGuinty hoped to acquire the extra seat he needed to restore his party's majority. The New Democrats won that contest, holding the Liberals to a minority.

As recently as February, when Ms. Wynne reached a deal with the OSSTF to end the feud, Mr. Coran crowed about his union's role in defeating the Liberals. In an internal memo to OSSTF members, Mr. Coran wrote that the union "stole the ability of the Liberals to form a majority government through our united actions and hard work in the Kitchener-Waterloo by-election on September 6, 2012."

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He also credited the OSSTF with helping push Mr. McGuinty to resign and making sure Laurel Broten, the education minister who imposed contracts on teachers, did not run to succeed him.

Just last month, London West NDP candidate Peggy Sattler tweeted that Mr. Coran was one of her "supporters."

Mr. Courtice said the party understands Mr. Coran had a job to do at OSSTF, and that his "strong, independent voice" would be an asset in the election.

"We want to put forward a candidate who's going to go to Toronto and is not going to be afraid to say what's on his mind and advocate for London," he said.

Before taking leadership positions in his union, Mr. Coran taught at secondary schools near London. He was elected OSSTF president in 2011.

Ms. Wynne has until Aug. 15 to call the by-election.

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With reports from Caroline Alphonso and Adam Radwanski