After suffering a setback at two Mercedes-Benz plants in Alabama on Friday, the United Automobile Workers union’s efforts to organize other auto factories in the South are likely to slow and could struggle to make headway.

About 56 percent of the Mercedes workers who voted rejected the U.A.W. in an election after the union chalked up two major wins this year. In April, workers at a Volkswagen plant in Tennessee voted to join the union, the first large nonunion auto plant in the South to do so. Weeks later, the union negotiated a new contract bringing significant pay and benefit improvements for its members at several North Carolina factories owned by Daimler Truck.

“Losing at Mercedes is not death for the union,” said Arthur Wheaton, director of labor studies at Cornell University School of Industrial and Labor Relations. “It just means they’ll have less confidence going to the next plant. The U.A.W. is in it for the long run. I don’t think they’re going to stop just because they lost here.”

Since its founding in 1935, the U.A.W. has almost exclusively represented workers employed by the three Michigan-based automakers: General Motors, Ford Motor, and Chrysler, now part of Stellantis. And it has long struggled to make headway at plants owned by foreign manufacturers, especially in Southern states where anti-union sentiment runs deep.

Workers at the Volkswagen plant had voted against being represented by the U.A.W. twice by narrow margins before the recent union win there. An effort a decade ago to organize one of the Mercedes plants failed to build enough support for an election.

Harley Shaiken, a professor emeritus at the University of California, Berkeley, noted that broad union organizing efforts seldom proceeded smoothly. In the 1930s, the U.A.W. won recognition at G.M. and Chrysler but struggled at Ford, which continued employing nonunion workers for a few years.

“I have no doubt they will continue organizing and eventually try for another vote,” he said.

In its past efforts in the South, the union was hampered by a negative image, which may have also played a part in the U.A.W.’s loss at Mercedes. For years, the three Michigan automakers were cutting jobs and closing plants, in part because of rigid and costly labor contracts. The union was also hurt by corruption cases that put several former senior officials, including two former U.A.W. presidents, behind bars.

Business leaders in Alabama ran a campaign against the U.A.W. that was based in part on the contention that the union was responsible for the decline of Detroit. In a January opinion essay published in The Alabama Daily News, the chief executive of the Business Council of Alabama, Helena Duncan, said the state would suffer the same fate if workers voted for the union.

“Much of the decay that exists in the ‘Motor City’ today results from untenable demands that the U.A.W. placed on its automobile manufacturers, an unwise move that sent untold numbers of jobs to right-to-work states like ours and crippled a once great metropolis,” Ms. Duncan wrote.

A year ago, the union elected a new president, Shawn Fain, who was untouched by the corruption scandals and vowed to take a more aggressive approach in contract talks. Then last fall, the union came away with substantial pay and benefit gains in negotiations with the Detroit automakers, after targeted strikes over some 40 days. Hundreds of Southern autoworkers began reaching out, asking for help organizing their nonunion plants. The U.A.W. responded by announcing that it would spend $40 million on organizing drives over the next two years.

“I’m not scared at all,” Mr. Fain said Friday in Alabama after the union lost the Mercedes vote. “I believe workers want unions, I believe they want justice, and we’re going to continue doing what we can do.”

Mercedes in a statement emphasized its direct relationship with workers and said it looked forward to making sure the company was “not only their employer of choice, but a place they would recommend to friends and family.”

The union has signaled that it expects to focus its organizing efforts on another Alabama plant — a Hyundai factory in Montgomery. But organizing that plant will probably be even harder than the campaign at the Mercedes factories, said Erik Gordon, a University of Michigan business professor who follows the auto industry.

The U.A.W. had allies at Volkswagen and Mercedes. Unions are powerful players in Germany, where those two companies are based. Under German law, worker representatives must occupy half the seats on a company’s supervisory board, the equivalent of an American board of directors.

Volkswagen and Mercedes both have groups called works councils through which managers and employees discuss and negotiate workplace issues and production plans. In its drive at the Volkswagen plant in Chattanooga, the U.A.W. had the support of the company’s works council and IG Metall, the powerful union that represents all German automotive workers.

The U.A.W. won’t have that kind of support at Hyundai’s Montgomery plant, Mr. Gordon said. “In general, Korean car companies have more adversarial relationships with unions than do the German manufacturers,” he said. “Korean companies are less used to sitting together in a conference room with unions.”

Last year, weeks after the U.A.W. won pay and benefit increases from the three Michigan-based automakers, Hyundai announced that it would increase its workers’ pay sharply over the next four years — a move widely seen as an attempt to dampen workers’ interest in joining the U.A.W.

“The decision to be represented by a union is up to our team members,” Hyundai said in a statement.

The Montgomery plant makes two popular sport utility vehicles — the Tucson and Santa Fe — and employs about 4,000 workers. An earlier U.A.W. drive to organize the plant in 2016 petered out without coming to a vote.

Last fall, the union said it planned to target plants owned by 10 foreign-owned automakers — Toyota, Honda, Hyundai, Nissan, BMW, Mercedes, Subaru, Volkswagen, Mazda and Volvo — and others owned by Tesla, which is based in Texas, and two smaller electric vehicle start-ups, Lucid and Rivian, both based in California.

The U.S. plants owned by those foreign and U.S. companies employ nearly 150,000 workers in 13 states, the union said.

In Alabama, however, the U.A.W. faced perhaps a more hostile environment than anywhere else. While it was campaigning at Mercedes, Gov. Kay Ivey spoke out against the union and headed a group of six Southern governors, all Republicans, who issued a letter suggesting unionizing could cause automakers to move jobs out of their states. One senior Alabama politician described the U.A.W. as “leeches.”

Mercedes brought in Nick Saban, the hugely popular former football coach at the University of Alabama, to talk to workers in an effort to persuade them to vote against the U.A.W.

Unions are traditionally seen as a Northern institution and are often linked with the civil rights movement, which alienates many people in Alabama, Mr. Gordon said. “It’s a very tough place for the U.A.W.,” he said.

That antipathy could also make it hard for the U.A.W. to negotiate contracts guaranteeing its members raises and other gains even if it wins unionizing votes. Lawmakers who oppose unions may put pressure on employers not to make big concessions in negotiations.

Mr. Fain and the U.A.W. have argued that unions are the best way for workers to demand higher wages when automakers are enjoying strong sales and profits in North America.

Public support of unions is stronger than it has been in years, including in the South. This year, 600 workers at an electric bus factory in Alabama voted to join the Communications Workers of America union. A week ago, they negotiated a new contract delivering pay raises and enhanced benefits.

The U.A.W. and other unions also have enjoyed the support of President Biden, who last fall joined striking autoworkers on a picket line in Michigan. The union endorsed Mr. Biden in this year’s election.

But that close association with the president may also hurt the U.A.W. with conservative workers in a Southern state who prefer Mr. Biden’s opponent — former President Donald J. Trump. Mr. Fain and Mr. Trump have often criticized each other, but polls have shown that a sizable minority of union households support the former president.