Workers at Mercedes-Benz's Alabama facilities have rejected a union drive in a setback to the United Auto Workers' ambitious campaign to organize the American South, according to results released Friday by US authorities.

A preliminary tally had 2,642 voting against unionization and 2,045 in favor, a margin of 56 to 44 percent, according to the National Labor Relations Board, which oversaw a five-day vote at the Mercedes auto manufacturing plant and battery complex near Tuscaloosa.

Led by President Shawn Fain, the UAW had high hopes for a second victory after winning an April election at Volkswagen in Tennessee -- the first successful union drive at a plant in a southern US state operated by a foreign automaker.

But in the election at Mercedes-Benz US International (MBUSI), UAW backers faced opposition not only from Mercedes itself, but from state and local officials who warned of job losses and depicted the campaign as a threat to the local economy.

"The workers in Vance have spoken, and they have spoken clearly!" Alabama's Republican Governor Kay Ivey wrote on X.

"Alabama is not Michigan, and we are not the Sweet Home to the UAW," said Ivey, who has fought unionization at Mercedes and at another plant operated by Hyundai.

Fain acknowledged disappointment in the result, and slammed Mercedes for what he called "egregious and illegal behavior" during the campaign, but insisted the UAW would press on with additional campaigns in the South.

"Justice isn't just about one vote or one campaign," Fain told a press conference.

"We're going to keep on fighting."

- David and Goliath? -

Mercedes, which replaced the CEO at MBUSI in late April, thanked employees who "made their voices heard on this important issue," according to a company statement.

"We look forward to continuing to work directly with our Team Members to ensure MBUSI is not only their employer of choice, but a place they would recommend to friends and family.

Union backers complained that the company subjected rank-and-file employees to a stream of anti-union communications at mandatory meetings.

Workers who supported unionization have filed unfair labor complaints with the National Labor Relations Board and with German officials, alleging the company flouted a law on supply chain practices.

"The contrast between Volkswagen and Mercedes is Mercedes did a full-out anti-union campaign," said Stephen Silvia, an economics professor at American University in Washington who has written extensively on unionization drives.

"Anti-union campaigns are effective," Silvia added.

The win at Volkswagen had raised hopes within the UAW about additional drives in the South at plants operated by Honda, Toyota, BMW and others.

The union campaign has garnered momentum after the UAW's strike last fall on Detroit automakers General Motors, Ford and Stellantis.

The strike resulted in large wage hikes and lifted the profile of Fain, who enlisted President Joe Biden to appear with auto workers on the picket line.

Robb Lett, a production team member in the Mercedes battery plant who has been part of the union organizing drive, told AFP the Detroit wins "gave us something tangible to point to."

"There are real things that have been won and we can win them too," he said.

But the American South has been a vexing target for organized labor for decades.

The UAW's progress has sparked heavy opposition from southern politicians, who argue the union will undermine the region's success in drawing large companies.

Fain likened the battle to a "David and Goliath" struggle, saying: "Sometimes Goliath wins a battle but ultimately David will win the war."