Some of the busiest ports in the U.S. faced crippling disruptions after about 45,000 longshoremen represented by the International Longshoremen’s Association (ILA) walked off their jobs at midnight after their contracts expired.

Talks over a new union contract between the ILA and the United States Maritime Alliance (USMX) collapsed on Monday and workers were expected to begin striking at 12:01 p.m. ET on Tuesday, striking 36 ports along the East and Gulf Coasts were affected.

As the Associated Press reported, workers began picketing the Port of Philadelphia shortly after midnight, walking in circles at a railroad crossing outside the port and chanting, “No work without a fair contract.” The Port of Virginia also confirmed the work stoppage.

The strike – the first longshore workers’ strike on the US East Coast since 1977 – threatens to close ports from Maine to Texas, crippling supply chains and weighing on the US economy.

Negotiators on both sides of the table accused each other of refusing to negotiate. The ILA has argued that USMX, which represents 40 maritime terminals and port operators, made “low-ball” offers of wage increases for workers and accused it of violating the previous contract by introducing automation at several U.S. ports.

It is estimated that the strike will cost the economy up to $5 billion per day. The union has said it will continue to carry military cargo and passenger cruise ships will not be affected.

USMX filed an unfair labor practice lawsuit against the union with the National Labor Relations Board on Wednesday, claiming the union is refusing to negotiate. Before Monday’s strike, USMX said it and the union had exchanged new wage offers. The union countered by claiming the accusation was a “publicity stunt.”

Current wages under the contract, which expired Monday, range from $20 an hour to the top wage of $39 an hour. The union is seeking a 77% pay increase over the course of the six-year contract, to a maximum rate of $69 an hour by 2030.

The Transportation Trades Department (TTD) of the AFL-CIO, the largest union federation in the United States, issued a statement in support of the union before the strike.

“Let us be clear: Employers, not workers, shirked their responsibilities and delayed labor negotiations until the eleventh hour, when the damage to the public and the national supply chain would be most devastating,” said Greg Regan and Shari Semelsberger. President and Secretary-Treasurer of TTD. “While USMX tries to shift the blame to the frontline workers moving our supply chain, they are to blame.”

The White House is monitoring the rift between the union and USMX and is urging both sides to reach an agreement. The AFL-CIO has urged House Republicans not to interfere and push for an injunction to block the strike. Joe Biden said ahead of the strike that he did not plan to intervene to block it.