The United Automobile Workers are pleased that the agreement will increase auto exports to Korea, and the United Food and Commercial Workers are encouraged that the pact will bolster meat exports to Korea. Both have embraced the deal, which is a modified version of an agreement that President George W. Bush first negotiated in 2007.

By supporting the agreement, the two unions are breaking with organized labor’s traditional opposition to free-trade agreements as a threat to American jobs. The machinists’ union has denounced the Korea deal, while two other powerful unions, the steelworkers and the communications workers, will announce their opposition on Thursday, union officials say.

Some trade experts say that the support of the autoworkers and the food and commercial workers is likely to sway enough Democrats in the House and Senate to ensure ratification. The agreement is also subject to approval by South Korea’s National Assembly.

“The ratification chances for the Korean agreement are extremely good because you now have a split in labor,” said Gary C. Hufbauer, a senior fellow at the Peterson Institute of International Economics.

Mr. Hufbauer said it was unusual for labor to be so divided over a trade agreement. Unified labor opposition has helped block ratification of trade agreements with Colombia, Panama and, until now, South Korea.

But facing pressure from the autoworkers and other unions, Mr. Obama pressed South Korea to renegotiate important provisions in the 2007 accord. The revised deal would reduce tariffs and other trade restrictions on American auto and beef exports and continue American tariffs on Korean cars and trucks for an extended period.

The U.A.W.’s president, Bob King, praised the agreement, saying it would significantly expand American auto exports to South Korea.

“I’m supporting this because I think it’s in the best interests of our members and in the best interests of our country,” he said in an interview on Wednesday. “The most important thing for the economic well-being of the U.S. is to protect jobs and create jobs, and I think this agreement will do that.”

The autoworkers are pleased that the deal keeps a 2.5 percent American tariff on Korean autos for four years and a 25 percent tariff on Korean sport utility vehicles for six years rather than lifting them immediately, as the 2007 accord called for.

Korea, meanwhile, will immediately cut its tariff on American auto imports in half, to 4 percent, while allowing in 75,000 American cars each year that meet American safety standards, even if they do not meet Korean safety standards.

Mr. King said the Obama administration had taken labor’s concerns into account more than any administration had in negotiating any previous trade agreement.

The United States had a $6.6 billion merchandise trade deficit with South Korea in the first nine months of this year, according to the Census Bureau, importing $35.5 billion in goods while exporting $28.9 million worth. In a study, the U.A.W. said there was an $8.9 billion trade deficit in automobiles with Korea last year, representing 70 percent of the bilateral trade deficit.

Denunciation of the U.A.W. by other unions and antitrade activists has been unusually harsh.

Jane Hamsher, a prominent left-wing blogger who founded Firedoglake, wrote that the U.A.W. was “a bunch of selfish pigs” for embracing the agreement and was a “Chinese-style union” that was “much closer to the interests of management and the government than those of its line workers.” She asserted that the U.A.W. embraced the accord in part to thank President Obama for rescuing General Motors and Chrysler.

The nation’s main labor federation, the A.F.L.-C.I.O., is usually quick to denounce trade agreements, but so far it has been silent about the Korean pact, apparently because of the disagreement among its member unions.

Several union officials said, however, that the A.F.L-C.I.O. would issue a statement early Thursday criticizing the Korean deal, in particular for not doing more to protect the rights of workers to unionize in both South Korea and the United States.

The Obama administration has hailed the agreement, which comes as the United States is showing support for South Korea after North Korea shelled a South Korean island, killing four people. The administration says the trade pact will further its goal of doubling exports within five years, while opening up South Korea’s $560 billion services market.

The administration pointed to a study by the International Trade Commission that predicted that American merchandise exports to Korea would increase to around $11 billion a year as a result of the agreement, while merchandise imports from Korea would grow about $6.7 billion.

Representative Sander Levin, a Michigan Democrat who is chairman of the House Ways and Means Committee and a highly influential voice among Democrats on trade, praised the agreement, calling it “a dramatic step toward changing from a one-way street to a two-way street for trade between the U.S. and South Korea.”

But some of the Democrats’ traditional union allies are unconvinced.

Thomas Buffenbarger, president of the International Association of Machinists, voiced concern that the agreement would increase imports of aircraft components from Korea, endangering the jobs of his union’s members. “We see it as just a repeat of Nafta,” he said, referring to the North American Free Trade Agreement, ratified in 1993, which removed many trade barriers among the United States, Mexico and Canada.

Mr. Buffenbarger said that the Korea agreement, like Nafta, promoted free trade, but not fair trade, by increasing imports made by lower-paid workers in other countries. “It helps corporations, but it really doesn’t help workers,” he said.

Some South Koreans are worried, too. Farmers there held a protest this week over the deal. The earlier version also drew opposition from Korean unions.

The Obama administration said the deal would help America’s farmers and ranchers by, among other things, eliminating Korea’s 40 percent tariff on beef imports.

The reduced tariffs on beef helped persuade the United Food and Commercial Workers to embrace the pact. “Academics estimate that the Korean agreement will create over 20,000 jobs in the U.S. meat export-producing sectors that employ hundreds of thousands of U.F.C.W. members,” the union said.

It also praised the deal for creating “a more level playing field” for the American auto industry.