(WASHINGTON) — Hundreds of thousands of rail workers stand on the brink of a nationwide strike that would paralyze the U.S. supply chain and passenger rail service ahead of the busy holiday season.

“Let me be clear: a rail shutdown would devastate our economy,” President Joe Biden said on Monday. “Without freight rail, many U.S. industries would shut down.”

Biden asked Congress to pass a law that imposes the terms of a tentative agreement reached in September but was rejected by several unions.

Still, the fate of a potential deal between rail workers and companies remains uncertain, especially if Congress fails to impose the tentative agreement. Four of 12 unions have already voted down the agreement and those four unions represent the majority of unionized rail workers.

A nationwide strike is expected unless the contract is ratified by each of the 12 rail unions, since all of the unions have vowed not to cross the picket line in the event of a work stoppage.

Unions have said rail employees are seeking improvements to working conditions, since workers do not receive paid sick days. The unions have accused rail companies of penalizing workers for taking time off for medical reasons and holding the nation’s economy hostage to ensure a favorable deal.

The National Carriers’ Conference Committee, or NCCC, which represents the nation’s freight railroads in national collective bargaining, has said rail employees are provided “significant” time off and the companies have offered a fair contract that includes a considerable wage increase.

Here’s a timeline of how the U.S. arrived at the precipice of a rail shutdown and what to expect as a deadline nears in the coming weeks:

July 15 – As rail unions and companies struggle to reach an agreement on the terms of a new contract, Biden signs an executive order that creates a presidential emergency board.

The board’s goal is to issue guideline recommendations for an agreement between the two sides in order to avert a potentially devastating rail strike.

Aug. 16 – The presidential emergency board issues recommendations for a compromise agreement between the unions and rail companies.

The recommendations include a 24% raise from 2020 to 2024 and bonus increases. The union’s demand for a new time-off policy, however, is omitted.

Sept. 14 – Rail workers and companies prepare for a potential strike that could cost $2 billion a day in lost economic output, according to the Association of American Railroads, which lobbies on behalf of rail companies.

Freight railroads are responsible for carrying 40% of the nation’s long-haul freight and a work stoppage could jeopardize these shipments.

Sept. 15 – Rail companies and unions reach a tentative labor agreement after 20 consecutive hours of negotiations brokered by U.S. Secretary of Labor Marty Walsh.

The tentative agreement “balances the needs of workers, businesses, and our nation’s economy,” Walsh said.

The agreement improves the time-off policies at the rail companies, which are a key sticking point in the negotiations, according to a statement from the two largest unions, the Brotherhood of Locomotive Engineers Trainmen, or BLET, and the SMART Transportation Division, or SMART-TD.

Oct. 11 – A union representing about 12,000 rail workers votes down the tentative contract that was brokered by the White House, raising the possibility of an eventual strike.

The Brotherhood of Maintenance of Way Employees Division of the Teamsters, or BMWED, rejects the tentative contract due to frustration with compensation and working conditions, particularly a lack of paid sick days, BMWED President Tony Cardell says in a statement.

“Railroaders do not feel valued,” Cardell says. “They resent the fact that management holds no regard for their quality of life.”

The National Carriers’ Conference Committee, or NCCC, the group representing the freight railroad companies, expresses “disappointment” in the decision to reject the contract.

Oct. 27 – A second union rejects the White House-brokered deal, elevating the likelihood of a nationwide strike.

The vote by the Brotherhood of Railroad Signalmen, a union representing 6,000 workers, centers on the lack of paid sick days, according to a statement from Brotherhood of Railroad Signalmen President Michael Baldwin.

The NCCC expresses disappointment over the union vote.

The tentative contract “included the largest wage package in nearly five decades, maintained rail employees’ platinum-level health benefits, and added an additional day of paid time off,” the NCCC says in a statement.

Nov. 21 – The nation’s largest rail union votes down the tentative contract brokered by the White House, dramatically escalating the likelihood of a strike.

SMART-TD, which represents about 28,000 conductors, narrowly rejects the contract in a vote that garnered record turnout, the union says.

The second-largest rail union, made up of engineers, votes in favor of the contract, splitting the top rail unions.

Nov. 28 – Biden asks Congress to intervene and avert a potential strike by forcing the workers’ unions to accept a White House-brokered deal as a December deadline approaches.

In a statement, Biden touts himself as a “pro-labor” president but says the larger economic implications outweigh those concerns.

“I am reluctant to override the ratification procedures and the views of those who voted against the agreement. But in this case — where the economic impact of a shutdown would hurt millions of other working people and families — I believe Congress must use its powers to adopt this deal,” he says.

He calls on Congress to quickly pass legislation to adopt the White House-brokered deal reached in September.

Outgoing Speaker Nancy Pelosi later says in a statement that the House will soon take up such legislation and will not modify the agreed-upon terms.

Nov. 29 – Biden tells ABC News he’s “confident” the U.S. will avert a rail strike.

“Congress, I think, has to act to prevent it,” he says. “It’s not an easy call, but I think we have to do it.”

Dec. 4 – By the end of the week that concludes on Dec. 4, House lawmakers are expected to take up a bill that adopts the tentative agreement brokered by the White House.

If passed, the bill will be sent to the Senate, where it faces uncertain prospects as the chamber is split 50-50 between the parties. Most legislation requires 60 votes to overcome a filibuster.

Dec. 9 – A strike deadline looms on Dec. 9, when unions could move forward with a coordinated work stoppage.

Additional reporting contributed by Morgan Winsor, Sam Sweeney, Sarah Kolinovsky, Amanda Maile, Zunaira Zaki, Trish Turner, Ahmad Hemingway and William Kim

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