The International Longshoremen’s Association strike is shaping up as a test of the president’s mettle on the eve of the election. The effort to shut down the ports and block American commerce is the kind of union overreach that, in 1947, led the GOP-controlled Congress, over President Truman’s veto, to enact the Taft-Hartley Act. That law would allow President Biden to reopen the ports while negotiations go on — if he has the aforementioned mettle.

So far, he’s vowing not to invoke Taft-Hartley. His weakness is a gift to the Longshoremen’s union, whose president, Harold Daggett, is crowing that the strike is a body blow to the economy and “is going down in history.” Vows he: “Nothin’s gonna move without us — nothing.” His words will provoke shudders for retailers stocking up for the holidays, as well as manufacturers in need of raw materials, and any business that has ties to global trade. And who doesn’t?

The Biden-Harris White House, though, has other considerations to bear in mind before quashing a disruptive strike. “It’s hard to exaggerate how politically toxic” such a move “would be to the Harris presidential campaign,” reckons the editor of the venerable Journal of Commerce, Mark Szakonyi. Vice President Harris has been scrambling to sew up union support, especially after the Teamsters declined to offer an endorsement.

The dockworkers’ strike, which threatens, according to J.P. Morgan, to cost between $3.8 billion and $4.5 billion per day also underscores the potentially awkward fit between an increasingly militant labor movement and President Trump’s wing of the Republican Party. Trump has made inroads with union voters, even inviting the Teamsters’ chief to speak at the GOP convention, but it’s hard to square a strike on this scale with a pro-growth agenda.

Mr. Biden, for his part, acceded to the White House in part on his pledge to be the “most pro-labor president ever,” which could make a resort to Taft-Hartley a less than palatable prospect. Even so, the president did intervene in 2022 to stop a railroad strike that threatened to further stoke the inflationary wave that his federal overspending had induced. Mr. Biden, in that case, acted under the Railway Labor Act and signed a law passed by Congress.

The labor action that began today — the first on the East Coast since one in 1977 that lasted 44 days — certainly threatens to “imperil the national health or safety.” That’s the legal standard in Taft-Hartley that gives the president the power to invoke the Act and set the strikers back to work. The Biden-Harris administration, though, has already proven that it’s willing to put the interest of the waterfront unions ahead of the public interest.

Mr. Biden showed his hand in 2022 when he failed to side with New York Harbor’s Waterfront Commission, an interstate compact between the Empire and Garden States, after New Jersey sought to torpedo the crime-fighting agency. The commission — created in 1953 as a result of the Sun’s Pulitzer-winning coverage of waterfront crime — ran afoul of the Longshoreman’s Union for its efforts to uphold the rule of law on the docks.

The New York Post warned that closing the commission would “hand the ports over to the mob.” It called to mind the bad old days when “thievery and skulduggery” prevailed on the waterfront, as a Sun editorial in 1948 put it, lamenting a “reign of lawlessness.” New York’s docks were ruled then by corrupt union officials like John “Cockeye” Dunn, who in 1949 went to the chair for murdering an uncooperative “hiring boss” on Manhattan’s Pier 51.

The Supreme Court sided with New Jersey, and the Biden-Harris administration, in 2023 and allowed the Waterfront Commission to lapse. The victory could well have emboldened the Longshoremen to mount their strike as they try to extort “pay back” from shipping firms. It took 44 days before President Carter stepped in to stop a destructive port strike four decades ago. How long will Mr. Biden and Ms. Harris dally before taking action on this head?