The United States, Japan and 10 other Pacific Rim nations on Monday reached final agreement on the largest regional trade accord in history, teeing up what could be the toughest fight President Obama will face in his final year in office: securing approval from Congress.

The conclusion of the Trans-Pacific Partnership, after years of negotiations and a series of sleepless nights here, was merely “an important first step,” conceded Michael B. Froman, the United States trade representative, as he and other weary officials announced their accord.

Now the deal faces months of scrutiny in Congress, where some bipartisan opposition was immediate. That debate will unfurl against the backdrop of a presidential campaign in which populist anti-trade talk against the deal is already prominent.

Still, for Mr. Obama the accord could be a legacy-making achievement, drawing together countries representing two-fifths of the global economy, from Canada and Chile to Japan and Australia, into a web of common rules governing trans-Pacific commerce. It is the capstone both of his economic agenda to expand exports and of his foreign policy “rebalance” toward closer relations with fast-growing eastern Asia, after years of American preoccupation with the Middle East and North Africa.

“When more than 95 percent of our potential customers live outside our borders, we can’t let countries like China write the rules of the global economy,” Mr. Obama said in a statement. “We should write those rules, opening new markets to American products while setting high standards for protecting workers and preserving our environment.”

Airports from Seattle to Amsterdam are reporting a falloff in cargo traffic to and from Asia, in what transportation executives and analysts say is a worrying sign for the health of global trade.

The data, released in recent days by the various airport authorities, marked the end a monthslong stretch of rapid growth in air-freight volume across the U.S. and Europe. The abrupt reversal demonstrated how rapidly economic problems in China and other emerging markets have reduced the flow of goods around the world.

China’s economy has stumbled this year. In early August, the country’s policy makers unexpectedly devalued the yuan, in hopes of boosting growth by reinvigorating exports. Chinese stock markets plummeted, triggering steep drops in share prices world-wide, amid fears that China’s woes would damp global growth.