WASHINGTON (Reuters) - Three Democratic U.S. senators on Tuesday questioned a decision by Volkswagen AG’s U.S. unit to delay a union election for workers at its Tennessee assembly plant.

The Volkswagen Chattanooga Assembly Plant in Chattanooga, Tennessee November 4, 2015. Volkswagen told NHTSA that it would recall about 92,000 vehicles, which are some 2015 and 2016 models of Jetta, Passat, Golf and Beetle, in the United States. REUTERS/Tami Chappell

Earlier this month, the largest German automaker won its bid to put off a union vote for 1,700 workers at the Chattanooga plant until its challenge to a smaller United Auto Workers bargaining unit at the factory is settled.

The National Labor Relations Board, in a 2-1 decision on May 3, granted Volkswagen’s motion to stay an election petition filed by some of its workers last month.

Senators Gary Peters and Debbie Stabenow of Michigan and Sherrod Brown of Ohio wrote to Scott Keogh, president of Volkswagen Group of America, on Tuesday, expressing “deep concern with delays” to the vote.

“We urge you to immediately drop any efforts to oppose or postpone the election,” the said.

UAW spokesman Brian Rothenberg urged VW to allow a vote of its workers: “Volkswagen should stop obstructing the rights of Chattanooga workers to vote and have a union – enough is enough.”

Volkswagen spokesman Mark Clothier confirmed the company had received the letter and would respond.

“We respect the decision of our team and their right to decide on representation. We have taken a neutral position on the issue and will continue to do so,” Clothier said.

In December 2015, 160 skilled trade maintenance workers voted to unionize and affiliate with the UAW, the union said.

VW declined to bargain with the union, saying the unit needed to include both skilled trade maintenance workers and production workers.

Volkswagen has stated it is neutral on workers joining a union but the senators said its “actions suggest otherwise.”

VW began production in 2011 at the plant, which builds the Passat car and the Atlas SUV. In January, VW said it was investing $800 million to build a new electric vehicle in Tennessee and add 1,000 jobs at the Chattanooga plant that will begin EV production in 2022.

The senators “have heard that facility supervisors in Chattanooga are engaging in direct anti-union conversations with workers in the workplace, including pulling workers off the production line to ask if they support the union,” they said.

In February 2014, workers at the plant narrowly voted against union representation, which had been seen as organized labor’s best chance to expand in the U.S. South.

UAW membership has plummeted 75 percent since 1979 and now stands at about 396,000. The UAW has failed for two decades to organize foreign automaker plants in the United States.