Volkswagen employees’ rejection of the United Auto Workers in Chattanooga, Tenn., signals that it will be difficult for unions to enter private-sector workplaces, particularly in the South, where organized labor has been making a concerted effort to grow over the long term.

The workers’ decision announced late on Friday to reject UAW membership at the three-year-old plant was a broad blow, as labor’s largest federation — the AFL-CIO — tries to reinvigorate a movement beset by long-term membership losses and eroding political clout. The vote went 712 to 626 against the union.

“It wasn’t just a loss for the UAW, it was a loss for the AFL-CIO and the entire labor movement,” said Gary Chaison, a professor of industrial relations at Clark University in Worcester, Mass. “They have a product they’re selling and people aren’t buying it.”

Last year, unions represented 11.3% of U.S. workers, flat with 2012 but down from about 20% in 1983. The private-sector membership rate was just 6.7%, compared with 35.3% in the public sector. Even union officials concede private-sector workplaces have been the most difficult to penetrate in recent decades.

More recently, unions’ political clout and financial coffers have suffered as they’ve fought bruising battles with lawmakers in cash-strapped states. Unions have marked some victories, but there have been losses in states that have laid off public-sector union members, curbed collective-bargaining rights and adopted right-to-work laws that allow employees to opt out of union membership and dues.

Amid membership declines at industrial unions like the UAW, the AFL-CIO has begun to partner with outside nonprofit groups and has tried to organize more low-wage service workers. The federation also announced a plan last year to start focusing more on Southern organizing and politics, particularly in Texas, which last year had one-fourth as many union members as New York state despite having 2.7 million more wage and salaried employees, according to Labor Department statistics.

The UAW launched its own southern strategy in recent years to organize foreign-owned auto makers throughout the traditionally antiunion region. But after this week’s Volkswagen vote, the prospect for gaining members at the UAW, and perhaps more broadly, looks bleaker now. This latest loss is also spurring debate over who is to blame for union defeats: the unions themselves or outside political forces that don’t want organized labor in their backyards. In Tennessee, politicians and out-of-state organizations mobilized against the Volkswagen vote.

“There is no use getting around it, it’s devastating,” Nelson Lichtenstein, a labor historian at the University of California at Santa Barbara, said of the Volkswagen vote. “Here’s a place where more or less the company was in fact genuinely neutral, and the union lost.”

In January, Germany’s Volkswagen AG and the UAW agreed on a plan to conduct the election. The company allowed union organizers inside the factory for more than a week, in an unusual case of an employer working with a union trying to gain a foothold in its workplace. Volkswagen cooperated with the UAW because the company wanted the plant workers to form a works council, which is a committee of employees who would give management feedback on running the plant. Works councils are the norm in Germany, and there are similar bodies in other European countries.

A group of Volkswagen workers opposed to the union launched their own campaign, and Republican politicians from Tennessee, including Sen. Bob Corker and Gov. Bill Haslam, urged workers to vote against the union on the grounds that unionization would raise costs and hurt the plant and the state competitively.

Sen. Corker issued a statement late on Wednesday saying that based on conversations he had, he felt “assured that should the workers vote against the UAW, Volkswagen will announce in the coming weeks that it will manufacture its new midsize SUV here in Chattanooga.” Volkswagen responded that there was “no connection” between the unionization and the decision on where to build the plant.

Immediately after the results of the three-day vote were disclosed, antiunion groups said it was a sign that workers see perils in unionizing and that the labor movement lacks value in the workplace.

“The workers in Chattanooga understood unionization would cost them hard-earned dollars, result in the loss of their individuality and imperil their future,” said Fred Wszolek, a spokesman for the Workforce Fairness Institute, one of organized labor’s most vocal opponents. The outcome “sends yet another message to Big Labor bosses that they cannot strong arm their way into American businesses and force workers into unions they don’t want,” Mr. Wszolek said.

Labor officials saw the results differently. In a brief statement Friday night after the votes were counted, the UAW said the election was met with “a firestorm of interference and threats from special interest groups.”

“While we certainly would have liked a victory for workers here, we deeply respect the Volkswagen Global Group Works Council, Volkswagen management and IG Metall for doing their best to create a free and open atmosphere for workers to exercise their basic human right to form a union,” said UAW President Bob King. IG Metall is a giant German industrial union.

Still, the vote loss won’t end the UAW’s attempts to organize at foreign-owned American plants. It has another long-running organizing campaign at a Nissan plant in Mississippi, among other efforts. But the loss in Tennessee will strip the union of some clout in future negotiations with the three Detroit auto makers and lead to more downward pressure on wages at unionized plants, said Mr. Lichtenstein, the labor historian at the University of California at Santa Barbara.

Mr. Lichtenstein said it is conceivable that the UAW could try to void the voting results by taking a challenge to the National Labor Relations Board, which oversees union elections, and allege that politicians and outside groups interfered with and tainted the election. But he said allegations of interference typically focus on the employer, which in this situation would likely make for a weak case, he added.

To be sure, the defeat could give labor some momentum by leading it to redouble efforts to organize more service-sector employees, such as taxi drivers and domestic workers, said Clark’s Mr. Chaison. Labor could do this through its partnerships with nonprofit groups and the so-called worker centers that don’t collectively bargain for workers but can provide certain other benefits, he said. “It has made the question of the new direction of the labor movement more relevant,” said Mr. Chaison.

Write to Melanie Trottman at and Kris Maher at

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