VANCE, Alabama, May 17 (Reuters) - The outcome of a vote on Friday by workers at a Mercedes-Benz factory in Alabama will be a key referendum on whether the United Auto Workers can maintain momentum in the historically anti-union South, but the union trails in early voting.

With votes representing almost half of eligible workers counted, employees at the plant were rejecting the UAW by a count of 1,451 to 1,073, according to a tally posted online by the UAW. About 5,100 workers were eligible to vote, and the tallies were being tabulated in batches so it was unclear how the voting will end.

Much of the politically conservative South has treated left-leaning unions as enemies, passing laws that make it difficult to operate, and anti-union forces have warned that companies are more likely to close union factories. A previous UAW corruption scandal that resulted in the arrest of several leaders further eroded support.

The UAW hopes to continue a run that includes an overwhelming organizing victory at a Volkswagen plant in Chattanooga, Tennessee, as well as a lucrative new contract at six Daimler Truck facilities across the South. Daimler Truck was spun off from what is now Mercedes.

A win at Mercedes would make it the second foreign-owned automaker in the U.S. South to join the UAW, a historic feat in a region that has previously been inhospitable to unions.

VW workers twice voted against the UAW before last month's win, and Nissan workers at a plant in Mississippi rejected the UAW by a wide margin in 2017. In 2021, workers at an warehouse in Alabama voted against forming a union by a more than 2-to-1 margin.

Results of the election are being overseen by the U.S. National Labor Relations Board.

Mercedes made its feelings clear in the run-up. Signs urging workers to vote "no" were hung around the plant, and the company hired anti-union firms to speak with workers about the potential risks of joining the UAW, according to workers, as well as photos and audio reviewed by Reuters.

Mercedes has rejected claims it prevented union organizing efforts in Alabama. A spokeswoman said the company respects employee unionizing efforts and is ensuring every worker has a chance to vote by secret ballot while having the information needed to make an informed choice.

Political opposition has been staunch in this campaign, too. Six U.S. governors, including Alabama's Kay Ivey, signed a letter asking workers to reject the UAW. They said unionization would stunt the auto industry's growth across the South.

Workers on both sides expect this election to be close. Mercedes employee Kay Finklea, who is pro-UAW, said the company's messaging, including a recent anti-union push with a local pastor, has swayed some to vote "no."

Clinching a win at Mercedes is a critical step in UAW President Shawn Fain's $40 million mission to organize more than a dozen automakers across the nation, including Toyota and Tesla. It would also allow the union to add to its dwindling ranks. (Reporting by Nora Eckert in Vance, Alabama Editing by Ben Klayman and Matthew Lewis)