While millions of Americans in Puerto Rico are without power, there’s a ship idling offshore filled with diesel fuel that could help alleviate their suffering. The ship, though, is legally prohibited from docking at Guayanilla because of a century-old law backed by Big Labor that many are calling obsolete and counterproductive.   

President Biden could help alleviate that suffering with the stroke of a pen, just as he did when he agreed to write off hundreds of billions of dollars in student loan debt for millions of other Americans, but the famously pro-union president has so far refused to do so. With each passing day, the chorus of complaints about his inaction is growing.

The reason the ship can’t offload its cargo can be traced back to the Merchant Marine Act of 1920. A section of the law known as the Jones Act, named after the congressman from Virginia who co-sponsored it, requires that goods being shipped within the United States, including territories such as Puerto Rico, be carried on ships made in America, owned by American companies, manned by American sailors, and flying the American flag.

The ship in question, the British Petroleum-owned GH Parks, which is carrying 300,000 gallons of diesel fuel that it onloaded at Texas City, Texas, sails under the flag of the Marshall Islands. For that reason alone, it has been stranded offshore, awaiting a one-time waiver of the Jones Act by the president.

Puerto Rico was pummeled by Hurricane Fiona on September 18, causing an island-wide blackout from which it is struggling to recover. As of Tuesday, about 30 percent of the island’s 1.47 million electric customers were without power and the demand for diesel fuel to power portable generators is off the charts.

Puerto Rico’s governor, Pedro Pierluisi, says he has been in constant contact with officials at Washington in recent days in an effort to secure a waiver for the GH Parks to unload its cargo, to no avail. Other officials from the territory, as well as lawmakers from areas on the mainland with significant Puerto Rican constituencies, also have been lobbying to have the restrictions lifted temporarily.

“I have asked for the personal intervention of the secretary of homeland security to allow a ship carrying diesel contracted by a private supplier that is near Puerto Rico to unload the fuel in order to help our people,” the governor said.

Labor unions, along with business groups representing American shipping interests, have long argued against issuing Jones Act waivers, even in cases of natural disaster. Mr. Biden, to the delight of union leaders, has often expressed support for the act, and even cited it when he signed a “Made in America” executive order during his first week in office. The order stipulated that any waivers to the act had to be approved by his “Made in America” office at the White House.

On Wednesday, one of those groups, the American Maritime Partnership, reiterated its opposition to a waiver and challenged what it called the “theatrics of a foreign oil company” trying to “take advantage of the crisis.” Puerto Ricans, the group said, have more than enough diesel fuel on hand without the cargo of the GH Parks.

“This stunt by a foreign oil company showing up unannounced in Puerto Rico while on its way overseas hoping to sell its fuel at a premium to Puerto Ricans in need, and thereby triggering a public and political rush to judgment, is bad precedent, a circumvention of U.S. law, and should never be tolerated,” the group said in a statement. “There continues to be absolutely no justification for a waiver of the Jones Act.”

Critics of the law, however, were hammering away at it Wednesday. Among the notables complaining about the administration’s lack of action a Democratic former treasury secretary and former president of Harvard University, Lawrence Summers. He took to Twitter Wednesday morning to ask why Mr. Biden has yet to sign off on the waiver.

The consequences of the act should be a warning to the Biden administration as adjusts American economic policies to boost domestic productivity and insulate supply chains from foreign influence, Mr. Summers said. The act was an attempt by President Wilson to boost American resilience and impose buy-American policies that has led to a number of unintended consequences.

“That it is increasing vulnerability, hurting [the] environment & leaving Puerto Ricans in the dark should make [the Biden administration] cautious & cost-benefit analysis focused as it implements industrial policies,” Mr. Summer wrote — in all-capital letters, no less, for maximum emphasis.