LANSING — An intensifying labor dispute could halt some Michigan highway construction projects as soon as Monday, a union official says.

The Operating Engineers Local 324 is mulling a strike against Rieth-Riley Construction Co., a large Indiana-based firm that is currently working on state projects on I-94 in western Michigan and I-75 near West Branch, in the northern Lower Peninsula, among other jobs, union spokesman Dan McKernan told the Free Press.

Rieth-Riley is the only major union contractor that did not sign a new contract with the Operating Engineers after a labor dispute that halted projects around Michigan for several weeks last fall, McKernan said.

About 200 of the union's members, who operate cranes and other heavy equipment, have been working on Rieth-Riley projects without a contract for close to a year, he said. Patience is running out, especially since some workers have received a $2-per-hour wage increase that was part of the new contract and others have not, he said.

"If the members decide they want to leave the jobs, how those projects would continue running without them is beyond me," he said.

Calls to Rieth-Riley officials late Thursday and Friday morning were not immediately returned.

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Unlike the labor dispute last fall, which the union described as an "involuntary layoff," and the contractors described as a "defensive lockout," any work stoppage would only impact Rieth-Riley projects, at least immediately, McKernan said.

But Rieth-Riley is one of the Michigan Department of Transportation's biggest contractors. Records show that MDOT currently has 89 open state and local contracts with the firm with a combined total value of $172 million. Those include projects on I-94 in Berrien County, near the Indiana state line, and projects on I-75 in both Ogemaw and Cheboygan counties in northern Michigan.

"I am not aware of any work being halted," said agency spokesman Jeff Cranson.

McKernan said Rieth-Riley is also a major supplier of aggregate and asphalt to other Michigan road construction firms, so the dispute has the potential to disrupt supplies of road-building materials to companies working on other Michigan road projects.

Central to the dispute last fall was the union's unwillingness to negotiate with the Michigan Infrastructure & Transportation Association — an umbrella group with both union and non-union firms among its members — on a new contract.

Since then, about 40 of the MITA contractors have formed a new group, the Michigan Union Contractors Group, and signed a new five-year agreement, McKernan said. The deal provides wage increases of $2 an hour in each of the first three years and increases of $1 an hour in each of the following two years, he said. It also clarifies contract language that says workers employed by subcontractors must receive the same wages, conditions and fringe benefits as workers employed by the prime contractor, he said.

The union has filed an unfair labor practices complaint with the National Labor Relations Board, McKernan said.

Contact Paul Egan: 517-372-8660 or pegan@freepress.com. Follow him on Twitter @paulegan4. Read more on Michigan politics and sign up for our elections newsletter.