The United Auto Workers still has options to unionize Volkswagen's Chattanooga car plant, labor law experts told Reuters.

While the UAW faces a one-year waiting period under U.S. labor law before it can hold another National Labor Relations Board election, it could hold a "private" vote that could include all plant workers or just some of them, said Wilma Liebman, former chair of the NLRB.

Also, under the so-called card check option, the union could present an employer with cards signed by workers expressing union support. VW officials have said they would prefer a secret ballot election, however.

The UAW lost a February election by a 53 percent to 47 percent margin. It dropped an appeal for a revote last week.