As workers at the Volkswagen plant in Chattanooga, Tenn., prepare to vote this week on whether to join the United Automobile Workers, they are facing unusual pressure from the state’s Republican legislators to reject the union.

State Senator Bo Watson, who represents a suburb of Chattanooga, warned on Monday that if VW’s workers voted to embrace the U.A.W., the Republican-controlled Legislature might vote against approving future incentives to help the plant expand.

“The members of the Tennessee Senate will not view unionization as in the best interest of Tennessee,” Mr. Watson said at a news conference. He added that a pro-U.A.W. vote would make it “exponentially more challenging” for the legislature to approve future subsidies.

A loss of such incentives, industry analysts say, could persuade Volkswagen to award production of a new S.U.V. to its plant in Mexico instead of to the Chattanooga plant, which currently assembles the Passat.