New York CNN Business  — 

Nearly 50,000 General Motors (GM) employees across the United States walked out late Sunday night after GM and the United Auto Workers union failed to reach an agreement on a new four-year contract.

Workers participating in the strike are spread across 31 General Motors factories and 21 other facilities in nine states.

Here’s what you may have missed:

Why this strike is a big deal

This is the US auto industry’s first strike in 12 years, and it’s also the biggest strike by any US labor organization in the same time span. GM, America’s largest automaker, has nearly 50,000 full-time and temporary UAW members on its payroll. The last autoworkers strike was also against GM, and lasted three days.

What workers want

The workers want higher hourly wages, lump sum payments and a better profit-sharing plan. They also want GM to agree to limit the use of temporary workers and give them a clearer path to permanent employment. The UAW says the two sides are far apart on other issues including health care benefits and job security.

Another big issue revolves around what will happen to two iconic assembly plants that are poised to close: One in Lordstown, Ohio, where work stopped earlier this year, and the Hamtramck plant in Detroit, which is scheduled to shut in early 2020. GM is offering to build an electric truck at Hamtramck and to make batteries for electric vehicles in Lordstown, according to a person familiar with the matter.

But even if the union agrees to those terms, work wouldn’t start immediately. The plants will remain dark for some time. Production could start sometime in the next four years.

What GM is offering

General Motors says it’s willing to work “around the clock” to reach a deal and end the walkout.

In a rare move for union negotiations, the automaker went public with its most recent offer. the automaker said it would commit to the following:

  • Preserving 5,400 jobs and investing $7 billion in its US plants
  • Offering wage or lump sum pay increases in all four years of the deal
  • A signing bonus of $8,000 per member
  • An improved profit sharing formula

CNN Business’ Christine Romans said GM’s transparency shows it is trying to “own the message.”

GM could be headed for junk bond status

Workers picket at a gate at the General Motors Flint Assembly Plant in Flint, Michigan.

General Motors’ credit rating could topple into junk bond status if the strike lasts more than a week or two, Moody’s said. GM’s debt rating is currently just one step above junk bond status.

Having its credit rating reduced to junk bond status would make borrowing money more difficult, and more expensive. GM was downgraded into junk bond status in 2005, several years before it ended up in bankruptcy court and in need of a federal bailout. It took until 2013 for it to shake off that rating, and have its debt classified as investment grade.

Dealerships aren’t hurting — yet

GM’s American plants are dark today, but the company still has plenty of cars to keep up with demand. In anticipation of a potential strike, it built up its supply of vehicles.

Although most car companies have a 53-day supply, GM has 59. GM also has an 80-day supply of trucks and SUVs, and an even larger supply of its most profitable trucks, like the Chevrolet Silverado and GMC Sierra.

– CNN Business’ Chris Isidore and Jordan Valinsky contributed to this report.