The ongoing union dockworkers' strike has left several U.S. ports on the East and Gulf coasts at a standstill for days, halting trade at the hubs that collectively handle about half of U.S. imports.

Major retail and business associations are sounding the alarm over the impact the work stoppage could have on the broader economy, and new data shows which companies have shipped the most goods into the affected ports over the past year.

According to the latest data from ImportGenius shared with FOX Business on Wednesday night, here are the companies that imported the most volume into East Coast and Gulf Coast ports from September 2023 to September 2024, by TEUs (Twenty-Foot Equivalent Units):

ImportGenius noted that these are "at least" estimates, saying the actual numbers are almost certainly higher, but companies can file with customs to have their identities redacted from granular shipping records. 

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William George, director of research for ImportGenius told FOX Business ships are continuing to pile up outside impacted ports, and what happens next is a "huge gamble" for carriers to go to the West Coast to unload their cargo with the uncertainty of how long the strike might last, in addition to other factors.

George likened the situation to shipping issues in the Suez Canal and Red Sea crisis, where carriers had to start rerouting ships when it became clear that there was going to be a substantial blockage – and that drives up shipping costs.

Paying for the additional fuel to move the freight further can add thousands of dollars to the cost of shipping a single container, he noted, and said other analysts that focus on spot rates are predicting 20% to 50% increases in ocean freight for impacted routes.

The International Longshoremen's Association (ILA), which represents 45,000 dockworkers, began its first strike since 1977 after its six-year contract with the U.S. Maritime Alliance (USMX), which represents port employers, expired Monday night.

Negotiations between the ILA and USMX have been deadlocked thus far over the union's demands related to wage hikes and compensation, as well as protection from automation at ports. 

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George told FOX Business, "Further complicating this is that there's no guarantee that if a ship is redirected and spends 15 days going to the West Coast, that it will be allowed to unload at LA and Long Beach, because the equivalent union on the West Coast historically has honored picket lines enforced by the ILA on the East Coast."

An analysis by JPMorgan estimated the daily cost of a port strike by East and Gulf Coast port workers to the U.S. economy between will be $3.8 billion and $4.5 billion per day as operations slow.

However, Anderson Economic Group (AEG), which specializes in economic impact estimates, expects the total cost of the strike to be much lower, at $2.1 billion for the first week.

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Still, Patrick Anderson, principal and CEO of AEG, told FOX Business he agrees with JPMorgan's analysts that the duration of the strike is likely to be determined by whether the Biden administration intervenes.