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.

US President Obama’s fellow Democrats last week shot down legislation crucial to finalizing the trade agreement on the grounds that it would hurt rather than help America. Unless he can convince scores of Democrats to change their votes in the coming days, the centerpiece of his much-touted re-engagement with Asia will slip away along with one of the last chances he has to leave his imprint on the world before leaving office.

“If the president cannot get” trade promotion authority “through Congress, it is a disaster for his Asia policy,” said Michael J. Green, a former Asia adviser to President George W. Bush and now at Georgetown University and the Center for Strategic and International Studies. “The administration will be dismissed as lame duck at a time when China is flexing its muscles.”

Moreover, Mr. Green and other analysts said, a failure to follow through on the trade deal would lead to Japan, Vietnam and other putative partners reversing course on economic reforms or tariff concessions required to join the multilateral trade zone with the United States, known as the Trans-Pacific Partnership, or T.P.P. And momentum may shift to economic institutions and agreements that do not include the United States, including the new Asian Infrastructure Investment Bank that China is creating over American resistance.

Congress is considering a trade bill that is pitting President Obama against many members of his own party and some Republicans. Though the two sides have major differences, a compromise is still possible and would be good for the American economy.

Mr. Obama is pushing for a bill that sets negotiating objectives for trade agreements and binds Congressional lawmakers to casting up-or-down votes on those deals for up to six years. This “fast-track” process would be used for two big pacts being negotiated now: the Trans-Pacific Partnership with 11 countries, including Australia, New Zealand, Canada, Japan and Mexico, and the Transatlantic Trade and Investment Partnership with the European Union. Mr. Obama argues that foreign negotiators will not put their best offer on the table if Congress can easily amend deals after they have been signed.

On Tuesday, the Senate is expected to take up the bill, which is opposed by some liberal Democrats who believe such trade agreements hurt American workers and by Republicans who do not want to give the president a victory. Getting support in the House might be even harder.

The important thing to remember about the Pacific and European trade deals is that they are not primarily about lowering customs duties and quotas. While these deals would reduce those barriers to trade, they would have much of their impact by getting countries to adopt similar regulations in areas like labor standards, environmental protection, how governments treat foreign investors and patent and copyright law.

Done right, the Pacific trade deal, which is nearing completion, could help reduce environmental destruction and improve the lives of workers in countries like Brunei, Peru, Chile and Vietnam, which are part of the negotiations. That agreement would also strengthen American alliances in Asia because it includes Malaysia and Singapore. Administration officials say other countries like South Korea and Thailand might want to join the pact in the future.

Some provisions that are expected to be part of the deal could raise problems. For example, some public interest groups fear that the deal could force developing countries to adopt strict patent regulations that could make many medicines unaffordable to poor people.