Canadian dairy farmers gathered in Ottawa to protest the Trans Pacific Partnership negotiations on Sept. 29. (Sean Kilpatrick/AP)

The Obama administration is aiming to wrap up talks this week on an expansive Asia-Pacific free-trade accord between the United States and 11 other nations, starting the clock toward a vote in Congress by early next year.

The prospect of the pact coming up on Capitol Hill in the midst of the U.S. presidential campaign has created fresh uncertainty about the prospects of success for the­ Trans-Pacific Partnership (TPP), which would represent the largest U.S. trade deal since the North American Free Trade Agreement in 1993. President Obama has called it central to his economic and foreign policy agendas.

Negotiators from the 12 nations are gathering in Atlanta with the goal of reaching a final agreement among countries that account for 40 percent of the world’s gross domestic product. Among them are Japan, Australia, Mexico and Canada. The pact would lower tariffs on beef, dairy products and automobiles, while setting up new regulatory provisions on pharmaceuticals, financial services and online commerce.

The White House had hoped to get the final deal to lawmakers by the end of this year to avoid the messy politics of an election season. But even if the administration breaks impasses on several thorny issues between the TPP countries and closes a deal this week, it is unlikely that trade legislation will reach the House and Senate floors for another three months at the earliest.

Opponents said they will renew an all-out effort to defeat the pact at a time when the nation’s growing income gap has become central to the 2016 race for the White House. Candidates including Republican front-runner Donald Trump and Sen. Bernie Sanders (I-Vt.), running for the Democratic nomination, have criticized U.S. trade policies, saying American workers are being put at a disadvantage. In recent days, Trump has called NAFTA “a disaster” and pledged to tear up the agreement or renegotiate it.

Appearing at the U.N. General Assembly in New York on Monday, Obama defended the TPP as part of his vision for U.S. leadership in which painstaking multilateralism triumphs over a “might makes right” approach to global affairs.

“We can promote growth through trade that meets a higher standard,” Obama said. “And that’s what we’re doing through the Trans-Pacific Partnership — a trade agreement that encompasses nearly 40 percent of the global economy; an agreement that will open markets while protecting the rights of workers and protecting the environment that enables development to be sustained.”

Administration officials are eager to get a deal in place to maintain momentum from the spring, when Congress granted Obama new fast-track powers to negotiate trade deals despite a ferocious backlash from Capitol Hill liberals, organized labor and environmental groups.

The fast-track trade authority sets up a vote on a specific timetable that cannot be subject to filibuster. But deep divisions remain among the TPP nations on dairy and sugar trade, automotive manufacturing and intellectual-property rights for pharmaceutical companies.

“The president has made clear that he will only accept a TPP agreement that delivers for middle-class families, supports jobs, and furthers our national security,” said U.S. Trade Representative Michael B. Froman, who will head to Atlanta on Wednesday. “That’s the bar we have to meet, and we won’t accept anything short of it. The substance of the negotiations will drive the timeline for completion, not the other way around.”

Administration officials are cautiously optimistic that they can close the gaps by week’s end, but their hopes of completing the pact over the summer were dashed when the talks broke off during the last negotiating round in Maui in July.

The AFL-CIO, which fiercely opposed the fast-track legislation, pledged to renew its fight against the trade pact. Once the TPP deal is finalized, the text of the agreement will be made public for the first time for at least 60 days before Obama can sign it, giving foes time to sow opposition to specific provisions.

“We’re going to go all-out on TPP,” said Thea Lee, the AFL-CIO’s deputy chief of staff. “As soon as the deal is done, we will ramp up education and mobilization.”

Obama has touted the deal as a cornerstone of his bid to integrate the United States into the economy of the vast and fast-growing Asia-Pacific region, where China has sought to expand its influence. China is not involved in the pact.

The issue is a rare one on which the White House is aligned with the Republican leadership in Congress. Obama won the fast-track powers on a bipartisan vote, with broad support from the GOP and the backing of only a fraction of Democrats.

The public is divided. About 49 percent of Americans viewed the TPP as a good thing for the United States, with 29 percent saying it was a bad thing, according to a Pew Research Center survey in June. Those surveyed in many of the other negotiating nations had higher opinions of the deal.

The debate over trade has put Democratic presidential front-runner Hillary Rodham Clinton in an awkward spot. She touted the TPP as the “gold standard” of trade agreements while serving as secretary of state during Obama’s first term, but she has hedged since declaring her candidacy for the White House.

But Clinton, concerned about Sanders’s rise in public opinion polls, has felt compelled to take stronger positions on issues on which she had previously equivocated, including announcing her opposition last week to the Keystone XL oil-sands pipeline.

Rep. Sander M. Levin (Mich.), ranking Democrat on the House Ways and Means Committee, sent a memo to colleagues last week outlining 13 areas of concern he had first raised in the spring, including on currency ma­nipu­la­tion and labor rights. Most of them remained unaddressed or were not resolved to his liking.

Public Citizen’s Global Trade Watch, another source of vehement opposition to the deal, noted in a memo for reporters that all of the House members are up for reelection next year, raising the specter that some who supported fast-track authority could look more skeptically at the TPP deal.