Nearly 13,000 United Auto Workers walked off the job after the deadline expired to land a new deal with the Big 3 U.S. automakers.

The "Stand Up Strike," is set to potentially become one of the largest in the industry's history, targeting not one but all of the "Detroit Three," the largest automotive manufactures in the country.

UAW members are currently on strike at three assembly plants in Michigan, Ohio and Missouri.

What is UAW?

The International Union, United Automobile, Aerospace and Agricultural Implement Workers of America, also known as the United Auto Workers, is a union with 400,000 active members and more than 580,000 retired members throughout the U.S., Canada and Puerto Rico.

The UAW has 600 local unions and represents workers across the industry, including multinational corporations, small manufacturers, state and local governments, colleges and universities, hospitals and private non-profit organizations, according to their website.

In 2019, the UAW went on strike, with 46,000 GM employees stopping work for 40 days, costing GM $3 billion.

UAW membership by year

UAW membership had fluctuated over the past 15 years, but is not nearly as high as historic membership levels. Nearly two decades ago, the UAW had more than 650,000 members. Its peak was 1.5 million in 1979.

In the past 10 years, union membership peaked in 2017 at 430,871 members and has slowly declined since.

Strike activity increases but union membership dwindles

In the first eight months of 2023, more than 323,000 workers walked off the job for better benefits, pay and/ or working conditions. But the rate of union members is the lowest its been in decades. In the 1950s, 1 in 3 workers were represented by a union. Now it’s closer to 1 in 10.

"Union density reached a high of over 30% in the post-World War II decades in the 1950s and 1960s," said Kent Wong, director of the UCLA Labor Center.

Why is union membership so low?

Labor laws in the U.S. make it more difficult for employees to form unions: More than two dozen states have passed "Right to Work" laws, making it more difficult for workers to unionize. These laws provide union representation to nonunion members in union workplaces – without requiring the payment of union dues. It also gives workers the option to join a union or opt out.

Even if workers succeed in winning a union election, it's a two-step hurdle, Wong said. "They have to prevail in an election to be certified as the bargaining unit representing the workers in any given a workplace. But beyond that, they have to get the company to agree to a contract.

Which states have the most union-represented employees?

Almost a quarter of workers living in Hawaii are represented by unions, according to the labor statistics bureau. At least 19 states have higher rates of employees represented by unions compared with the national average. South Carolina had the lowest rate of union represented employees at 2%.

UAW strike:Workers at 3 plants in 3 states launch historic action against Detroit Three

Explainer:Here's why the US labor movement is so popular but union membership is dwindling