Port operators and Eastern longshoremen agreed Friday to avoid a potentially crippling strike set for Sunday at 14 major US ports, at least for now. Negotiators refused to release details of the deal, but labor experts suggest the daring strike threat by dockworkers is indicative of a broader gambit by a besieged labor movement to claw back some power amid a strengthening US economy.

The agreement over so-called “container royalties” worth up to $15,000 a year for an average longshoreman does not fully resolve the dispute, but is part of a 30-day contract negotiation extension agreed upon by the International Longshoremen’s Association and the US Maritime Alliance, which represents shipping companies and ports.

“The container royalty payment issue has been agreed upon in principle by the parties, subject to achieving an overall collective bargaining agreement,” said George Cohen, director of the Federal Mediation and Conciliation Service.

While federal mediators refused to disclose the agreement and said “significant issues remain in contention,” Mr. Cohen said what he can report “is that the agreement on this important subject represents a major positive step toward achieving an overall … agreement.”

With Washington frantically trying to stave off a national “fiscal cliff” of tax increases and automatic budget cuts, the already wobbling US economy likely would have teetered further if the 14,500 longshoremen had walked off docks from New York to Houston on Sunday. The workers handle 40 percent of US container traffic – about 100 million tons a year – and a strike could have cost $1 billion a day by blocking what is, in effect, the lifeblood of the US marketplace.

The situation had become serious enough for state leaders, including Florida Gov. Rick Scott, to call on President Obama to invoke the Taft-Hartley Act, a federal labor law used by President George W. Bush to end a 10-day, 29-port lockout on the West Coast in 2002.

The gritty longshoremen, meanwhile, are uniquely situated to push their agenda, even at the risk of losing public and political support, labor experts say. While relatively small, the longshoremen’s union historically has been aggressive in protecting its workers’ benefits, and its members’ central role to the US economy makes the union a particularly tough negotiator.

The strike reprieve was in part achieved with the help of federal negotiators dispatched by Mr. Obama, who owes much of his political success to unions but who has been wary of pursuing pro-union legislation that could negatively impact the soft economy.

Citing traffic-clogging Thanksgiving protests by the Service Employees International Union at Los Angeles International Airport, bakers going to battle with Hostess, and a recent eight-day clerical workers strike at West Coast ports (where most longshoremen refused to cross picket lines), labor expert Philip Dine says the movement “is clearly in a more aggressive mode right now.”

“I think what we’re seeing is … labor realizing that it needs to think about and pursue its own goals,” says Mr. Dine, author of the newly revised book “State of the Unions.” “And when you’re willing to [cripple port traffic and] stall traffic at major airports on the busiest travel day of the year, it shows labor is willing to antagonize shoppers and travelers, and it means labor means business.”

Chris Rohmberg, a sociology professor at Fordham University, told The Oregonian newspaper this month that a series of dockworker actions – including the recent clerical strike and a looming shutdown of several northwestern ports over failed contract negotiations – are about workers setting a new tone with their employers as the economy struggles its way out of recession.

“The whole relationship between employers and workers is on the table,” Mr. Rohmberg told The Oregonian.

While union membership has been falling for decades, several dockworkers’ unions have seen membership tick up in recent years, partly in response to a recession and prolonged recovery that has made workers feel particularly vulnerable to management cutbacks.

According to Dine, discord in the labor movement and disillusionment over broader social and economic trends have been largely cast aside amid what many see as an all-out war on labor by conservatives, including nearly a dozen Republican governors in the Rust Belt. The governors are seen as trying to eviscerate the labor movement by arguing, in essence, the illegitimacy of public worker unions, whose members now make up the majority of the US labor movement.

“At this point, if you destroy public-sector unions you destroy the labor movement, which means labor is now fighting for its life,” says Dine. “As a response, the labor movement has become more serious, determined, unified, and energetic, and so it’s not really any surprise that you’re seeing these signs of activism, which, objectively, you might not expect in a poor economy or with a Democratic president.”

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While negotiators remain “cautiously optimistic,” in Cohen’s words, a longshoremen’s strike remains possible. Such an action would have a vast impact on the US economy, crippling a US supply chain that has become increasingly “direct-to-market” as retailers and car companies rely less on stockpiled goods.

Meanwhile, labor experts say consumers and businesses should expect to see more labor disruptions across the country, as unions begin to rally after years of retrenchment.