SINGAPORE — Your GAP shirts and Nike shoes might get cheaper, but the jobs of the 1,350 U.S. employees of Boston-based sneaker-maker New Balance could be in trouble.

Detroit automakers might get to sell more cars and trucks into Japan, but the U.S. manufacturers of the fabric used in those vehicles could be out of work.

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These are among the dozens of tradeoffs in play as officials from the United States and 11 other countries descend on Singapore this weekend to negotiate what would be the largest trade deal in world history — potentially 1.5 times the size of NAFTA and worth as much as $124 billion in new U.S. exports by 2025.

While thousands of U.S. jobs are on the line, what’s also at stake is President Barack Obama’s “Asia pivot,” as administration officials believe a breakthrough on the Trans-Pacific Partnership could seal the president’s legacy on trade and reverberate in the global economy for a generation.

The negotiations started Saturday, but intense talks were well under way at the World Trade Organization’s meeting this week in Bali, Indonesia. U.S. negotiators were spied huddling one-on-one with counterparts from New Zealand and other countries in hotels and restaurants, working through the details of the mega-regional trade deal.

However, after nearly four years and 19 rounds of talks, the deal’s finish line remains far away. U.S. Trade Representative Michael Froman’s team headed to Singapore will again confront deep disputes with Japan, Australia, Vietnam and other countries — all with their own political pressures at home — this week. Japan, for example, won’t budge on allowing other countries to sell more rice and beef there. Australia wants to ditch the limits Congress placed on the sugar it can sell in the United States, and Vietnam is furious over congressional efforts to block its catfish exports.

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“For the countries involved, the decisions and the compromises that need to be made are very sensitive and very difficult,” Vice President Joe Biden said this week in Tokyo after visiting Japanese Prime Minister Shinzo Abe.

“The upsides of getting such an agreement done are almost beyond comprehension. They’re incredibly positive for all countries involved. But the reward does not diminish the realization of how difficult the compromises needed are,” Biden said.

The merits of the deal are up for debate, but Biden isn’t overselling its scope. Twenty years after NAFTA was implemented, the Pacific Rim deal could dwarf it, with the countries involved accounting for 40 percent of global GDP.

“It is deepening our economic ties in the region,” Jeffrey Schott, a senior fellow at the Washington, D.C.-based Peterson Institute for International Economics who was in Indonesia this week for the WTO meeting, said of the deal. “But with a view that this group [of participating countries] is going to expand and hopefully encompass the entire Asia-Pacific region.”

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The Pacific Rim pact could benefit Americans in many consequential ways.

Farmers see new chances to export their rice, beef, soybeans and more to Pacific Rim countries, while California rice growers and Western state ranchers also see the deal as a huge opportunity.

“We can produce more than can be absorbed at home, so having access to the fastest-growing markets in the world in the Asia-Pacific region is going to be a very, very positive boost,” said Calman Cohen, president of the Emergency Committee for American Trade.

Automakers hope they can sell more of their cars and trucks to a notoriously closed-off Japan. And the booming U.S. natural gas market would find an enormous customer in Japan, allowing even more production at shale formations in Texas and the Plains.

But business officials say it’s about more than market access. “In a lot of cases, our business was better where the U.S. was deeply engaged — where there were U.S. style rules and U.S. influence in the region,” said Scott Miller, who was Procter & Gamble’s global trade policy director for 15 years.

The deal could also make a difference in other areas. The pact could boost U.S. efforts to reach international social goals. Vietnam, for example, could be forced to strengthen its lax rules against child labor, to improve factory safety and to begin allowing unions to form. It also gives the United States a venue to push for useful tweaks to other countries’ laws and regulations — and vice versa. Movie-makers and musicians could get a boost through a crackdown on Asian market rip-offs.

Pharmaceutical drugmakers, for example, see longer patents in Asian countries as a boon to their bottom lines — a controversial aspect itself. “The U.S. government seems determined to give pharmaceutical companies more power to raise the cost of medicines for millions of people around the world, while curtailing the power of governments to protect public health,” said Rohit Malpani, Doctors Without Borders’ director of policy and analysis.

But the Trans-Pacific Partnership also brings with it more than a few worries for the United States, especially those who represent labor interests.

The AFL-CIO fiercely opposes the pact, fearing it would cost hundreds of thousands of manufacturing jobs. The domestic textile industry, which in the 1790s began the transformation from an agrarian economy and has already shrunk from a peak of 3 million U.S. jobs to 500,000 today, is worried, too.

The 5,000 workers at North Carolina-based Parkdale Mills’ 28 U.S. plants are fretting that the Pacific Rim pact could deal their industry a fatal blow — forcing career textile tradesmen into a desperate job search. “They have master’s degrees in running machinery” but often no college education, said Dan Nation, the company’s president of international operations. “It’s a blue-collar job.”

“Once you lose jobs to Asia, you don’t get them back. They’re gone,” Nation said.

Winners and losers could all be determined — or at least close to determined — in Singapore. But deal or no deal, the drama won’t end there. Congress must approve whatever Obama’s administration sends them in order for it to take effect.

That’s no cinch.

Rep. George Miller of California, the top Democrat on the House Education and the Workforce Committee, said recent trade deals with countries like Colombia have failed to improve the way workers and labor unions are treated — and that the Pacific Rim deal is troubled, too.

“These countries are much more hostile to labor than even Colombia,” he said. “We see a road map to failure.”

More than 175 House members have signed letters to Obama warning that they are likely to oppose giving him a key legislative mechanism to get the deal through without amendments.

Advocates of free trade, meanwhile, are already rallying support.

“We need to be sharing, engaging, visiting, building friendships and relationships with other countries,” said Rep. Dave Reichert (R-Wash.), a member of the House Ways and Means Committee, which handles trade issues.

“Those who are concerned about human rights issues and fair treatment of employees — we could have a huge impact on influencing positive behaviors in those areas,” he said.

Eric Bradner reported from Washington. Doug Palmer reported from Bali, Indonesia.