VANCE, AL—Workers at the Mercedes-Benz assembly plant here began voting Monday on whether to join the United Auto Workers union, a significant test of whether the labor group can maintain momentum in the historically anti-union South.

The vote comes just weeks after workers at Volkswagen’s assembly plant in Chattanooga, TN, voted overwhelmingly in favor of UAW representation. A second UAW victory could pave the way for the union to organize more than a dozen additional factories and add to its dwindling ranks.

The organizing campaign at the Mercedes factory has been much more contentious than the drive at VW. The company has posted signs and fliers urging workers to vote no. Mercedes also hired anti-union firms to speak with workers.

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.

The 5,200 employees at the assembly plant and a nearby battery factory will cast ballots this week, with final results expected Friday.