Unionized workers at local automotive companies are gearing up for a massive general strike in the coming weeks, with the largest Hyundai Motor Union expected to join the plan, the first in four years.

The Korean Metal Workers Union, which includes the car industry, started a two-day vote on Tuesday to ask the approval of their 150,000 members on a four-hour walkout plan on July 13 and 20.

The 45,000-member Hyundai Motor Union, which makes up about 70 percent of the total metal union members, also carried out its own vote on the day. Its participation is highly likely as union members have thus far never voted down on a strike plan.

With the participation of the powerful union of the nation’s largest carmaker, observers say, this year’s “summer labor strike,” or the seasonal wave of labor strikes, could be the fiercest in recent years.

The Hyundai Motor Union had avoided strikes for the past three years as dovish leaders led collective bargaining with the management without strikes. But now with hawkish leaders in control, it is back on the offensive.

The company had already filed a grievance with the Central Labor Relations Committee early this month about the reasonability of the strike plan, with the final decision coming out possibly on Wednesday.

“Following the result, we will respond strictly to illegal collective action. We think there should be political consideration of the looming presidential election,” said a Hyundai Motor spokesperson.

“As we are launching new models recently, it seems unavoidable for our customers to face some inconvenience in getting their new cars.”

The 30,000-member Kia Motors Union, which has avoided strikes over the past two years, also started a vote on the strike plan.

Meanwhile, union members of GM Korea, the local unit of U.S. automaker General Motors, carried out three-hour strikes on Tuesday, with a recent approval of a partial strike plan.

After a series of collective wage-bargaining talks since May, the management and labor failed to reach an agreement.

Industry sources said the strikes would largely affect the carmaker’s exports to overseas markets. Of the Chevrolet Cruze and Aveo vehicles sold in Europe, 98 percent are produced in Korea.

The union of Kumho Tires Co., the country’s second-largest tire maker, also conducted separate partial strikes on the day as well.

Meanwhile, the umbrella Metal Workers Union has been hinting at a general strike in August unless any tangible agreement is reached on key labor issues including irregular workers, layoffs and the amendment of basic labor laws.

By Lee Ji-yoon (jylee@heraldcorp.com)