DETROIT (AP) — The United Auto Workers union faces the latest test of its ambitious plan to unionize auto plants in the historically nonunion South when a vote ends Friday at two Mercedes-Benz factories near Tuscaloosa, Alabama.

The voting at the two Mercedes factories — one an assembly plant, the other a battery-making facility — comes a month after the UAW scored a breakthrough victory at Volkswagen’s assembly factory in Chattanooga, Tennessee. In that election, VW workers voted overwhelmingly to join the UAW, drawn by the prospect of substantially higher wages and other benefits.

The UAW had little success before then recruiting at nonunion auto plants in the South, where workers have been much less drawn to organized labor than in the traditional union strongholds of Michigan and other industrial Midwest states.

A victory at the Mercedes plants would represent a huge plum for the union, which has long struggled to overcome the enticements that Southern states have bestowed on foreign automakers, including tax breaks, lower labor costs and a nonunion workforce.

Some Southern governors have warned voting for union membership could, over time, cost workers their jobs because of the higher costs that the auto companies would have to bear.

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Yet the UAW is operating from a stronger position than in the past. Besides its victory in Chattanooga, it achieved generous new contracts last fall after striking against Detroit Big 3 automakers: General Motors, Stellantis and Ford. Workers there gained 33% pay raises in contracts that will expire in 2028.

Top-scale production workers at GM, who now earn about $36 an hour, will make nearly $43 an hour by the end of their contract, plus annual profit-sharing checks. Mercedes has increased top production worker pay to $34 an hour, a move that some workers say was intended to fend off the UAW.

Shortly after workers ratified the Detroit contract, UAW President Shawn Fain announced a drive to organize about 150,000 workers at more than a dozen nonunion plants, mostly run by foreign-based automakers with plants in Southern states. In addition, Tesla’s U.S. factories, which are nonunion, are in the UAW’s sights.

About 5,200 workers at the Mercedes plants are eligible to vote on the UAW, the union’s first election there. Balloting is being run by the National Labor Relations Board.

The union may have a tougher time in Alabama than it did in Tennessee, where the UAW had narrowly lost two previous votes and was familiar with workers at the factory. The UAW has accused Mercedes of using management and anti-union consultants to try to intimidate workers.

In a statement Thursday, Mercedes denied interfering with or retaliating against workers who are pursuing union representation. The company has said it looks forward to all workers having a chance to cast a secret ballot “as well as having access to the information necessary to make an informed choice” on unionization.

If the union wins, it will be a huge momentum booster for the UAW as it seeks to organize more factories, said Marick Masters, a professor emeritus at Wayne State University’s business school who has long studied the union.

“The other companies should be on notice,” Masters said, “that the UAW will soon be knocking at their door more loudly than they have even in the recent past.”

If the Mercedes workers reject the union, Masters expects the UAW leadership to explore legal options. This could include arguing to the National Labor Relations Board that Mercedes’ actions made it impossible for union representation to receive a fair election.

Though a loss would be a setback for the UAW, Masters suggested it would not deal a fatal blow to its membership drive. The union would have to analyze why it couldn’t garner more than 50% of the vote, given its statement that a “supermajority” of workers signed cards authorizing an election, Masters said. The UAW wouldn’t say what percentage or how many workers signed up.

A UAW loss, he said, could lead workers at other nonunion plants to wonder why Mercedes employees voted against the union. But Masters said he doesn’t think an election loss would slow down the union.

“I would expect them to intensify their efforts, to try to be more thoughtful and see what went wrong,” he said.

If the UAW eventually manages to organize nonunion plants at Hyundai, Kia, Nissan, Toyota and Honda with contracts similar to those it won in Detroit, more automakers would have to bear the same labor costs. That potentially could lead the automakers to raise vehicle prices.

Some workers at Mercedes say the company treated them poorly until the UAW’s organizing drive began, then offered pay raises, eliminated a lower tier of pay for new hires and even replaced the plant CEO.

Other Mercedes workers have said they prefer to see how the company treats them without the bureaucracy of a union.

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Chandler reported from Montgomery, Alabama.