DETROIT (Reuters) - Factory workers at Mercedes Benz's assembly plant in Alabama are moving forward with efforts to join the United Auto Workers union, and they plan to file a petition as soon as this week with U.S. regulators, three people familiar with the matter said on Tuesday.

Employees at the SUV plant in Vance, Alabama, plan to file paperwork with the National Labor Relations Board (NLRB) seeking a formal election to join the UAW, said the sources, who asked not to be identified as the timing is still fluid. The date of an actual vote is not yet certain.

A union spokesman declined to discuss a Mercedes vote time frame, but the UAW said in late February that a majority of about 6,000 workers there had signed cards to join the union.

President Shawn Fain is leading an unprecedented organizing effort for the 88-year-old UAW, endeavoring to unionize more than a dozen automakers, including Tesla, across the U.S.

The union has failed several times over the last two decades to organize U.S. facilities owned by VW and Nissan, but Fain hopes to succeed after reaching new labor deals last fall with the Detroit Three automakers: General Motors, Ford and Chrysler parent Stellantis.

Fain, who took on the head job at the UAW a year ago, says this time will be different, citing a more emboldened U.S. labor force that in part fueled the historic wins in Detroit.

A vote at Mercedes would follow a similar push at Volkswagen's assembly plant in Chattanooga, Tennessee, where voting on whether to join the UAW is scheduled to end on April 19.

An NLRB spokesman said the agency has received several unfair labor practice charges filed by the UAW against Mercedes, but has not yet received a petition for an election at the Alabama plant.

A Mercedes spokeswoman said the German company has not interfered with or retaliated against any team member seeking union representation, and that it prefers to maintain direct communication with employees.

For the UAW, expanding beyond the Detroit Three is the goal, starting with VW and Mercedes.

"When we return to the bargaining table in 2028, it won't just be with the Big Three, but with the Big Five or Big Six," Fain said in November.

The union has failed twice before to organize the VW plant in Tennessee. Efforts at other nonunion plants are ongoing and are expected to accelerate if the union wins early votes, the sources said.

A successful organizing campaign outside of Detroit would reverse declines in membership, which has dwindled from a high of 1.5 million UAW members in the 1970s to 370,000 last year, its lowest level since 2009.